Kernel wise there was quite some change around that time, as the 5.11 was being sunset, 5.13 got the new default and 5.15 was made available opt-in as preview, so depends on how frequent do you upgrade and if you explicitly opted in the new kenerl.
5.15.5-1-pve
got uploaded to pve-enterprise on 2021-12-06 if one was already on the then rather fresh 5.15 release, which was then still opt-in), for the 5.13 we got 5.13.19-2-pve
around that time (initial upload to testing at end of November) and the EOL 5.11 series got its last release out on 2021-11-07 as 5.11.22-7-pve
.
FWIW, you could check your setups /var/log/apt/history.log
, and its log-rotated variants, for what got newly in on your setup around that time.
OK, these are my logs on that cluster in production (production activation day 1 October 2021, all nodes restarted for pre-production tests, officially in production on the afternoon of 1 October 2021).
*********************************
Start-Date: 2021-10-04 21:30:06
Commandline: apt dist-upgrade
Upgrade: qemu-server:amd64 (7.0-13, 7.0-14), pve-container:amd64 (4.0-9, 4.0-10), libpve-common-perl:amd64 (7.0-6, 7.0-9), pve-kernel-helper:amd64 (7.0-7, 7.1-2)
End-Date: 2021-10-04 21:30:10
Start-Date: 2021-10-12 07:40:45
Commandline: apt dist-upgrade
Install: pve-kernel-5.11.22-5-pve:amd64 (5.11.22-10, automatic)
Upgrade: reportbug:amd64 (7.10.3, 7.10.3+deb11u1), libperl5.32:amd64 (5.32.1-4+deb11u1, 5.32.1-4+deb11u2), libpam-runtime:amd64 (1.4.0-9, 1.4.0-9+deb11u1), krb5-locales:amd64 (1.18.3-6, 1.18.3-6+deb11u1), libgssapi-krb5-2:amd64 (1.18.3-6, 1.18.3-6+deb11u1), pve-firmware:amd64 (3.3-1, 3.3-2), perl:amd64 (5.32.1-4+deb11u1, 5.32.1-4+deb11u2), python3-reportbug:amd64 (7.10.3, 7.10.3+deb11u1), libkrb5support0:amd64 (1.18.3-6, 1.18.3-6+deb11u1), libc6:amd64 (2.31-13, 2.31-13+deb11u2), locales:amd64 (2.31-13, 2.31-13+deb11u2), libkrb5-3:amd64 (1.18.3-6, 1.18.3-6+deb11u1), libpam-modules:amd64 (1.4.0-9, 1.4.0-9+deb11u1), base-files:amd64 (11.1, 11.1+deb11u1), libk5crypto3:amd64 (1.18.3-6, 1.18.3-6+deb11u1), rsync:amd64 (3.2.3-4, 3.2.3-4+deb11u1), libpam-modules-bin:amd64 (1.4.0-9, 1.4.0-9+deb11u1), perl-base:amd64 (5.32.1-4+deb11u1, 5.32.1-4+deb11u2), libpam0g:amd64 (1.4.0-9, 1.4.0-9+deb11u1), libc-l10n:amd64 (2.31-13, 2.31-13+deb11u2), libc-bin:amd64 (2.31-13, 2.31-13+deb11u2), pve-kernel-5.11.22-4-pve:amd64 (5.11.22-8, 5.11.22-9), pve-kernel-5.11:amd64 (7.0-7, 7.0-8), perl-modules-5.32:amd64 (5.32.1-4+deb11u1, 5.32.1-4+deb11u2)
End-Date: 2021-10-12 07:42:22
Start-Date: 2021-10-12 20:09:07
Commandline: apt-get dist-upgrade
Upgrade: pve-firewall:amd64 (4.2-3, 4.2-4)
End-Date: 2021-10-12 20:09:12
Start-Date: 2021-10-21 07:09:04
Commandline: apt dist-upgrade
Upgrade: libproxmox-acme-perl:amd64 (1.3.0, 1.4.0), libpve-storage-perl:amd64 (7.0-11, 7.0-12), proxmox-backup-file-restore:amd64 (2.0.9-2, 2.0.11-1), libpve-access-control:amd64 (7.0-4, 7.0-5), pve-container:amd64 (4.0-10, 4.1-1), libproxmox-acme-plugins:amd64 (1.3.0, 1.4.0), proxmox-backup-client:amd64 (2.0.9-2, 2.0.11-1), libpve-http-server-perl:amd64 (4.0-2, 4.0-3), libpve-common-perl:amd64 (7.0-9, 7.0-10)
End-Date: 2021-10-21 07:09:10
Start-Date: 2021-10-27 08:33:13
Commandline: apt-get dist-upgrade
Upgrade: tzdata:amd64 (2021a-1+deb11u1, 2021a-1+deb11u2)
End-Date: 2021-10-27 08:33:14
Start-Date: 2021-10-29 07:25:06
Commandline: apt upgrade
Upgrade: bind9-host:amd64 (1:9.16.15-1, 1:9.16.22-1~deb11u1), bind9-dnsutils:amd64 (1:9.16.15-1, 1:9.16.22-1~deb11u1), bind9-libs:amd64 (1:9.16.15-1, 1:9.16.22-1~deb11u1)
End-Date: 2021-10-29 07:25:07
*********************************
Start-Date: 2021-11-09 21:22:09
Commandline: apt upgrade
Upgrade: libldb2:amd64 (2:2.2.0-3.1, 2:2.2.3-2~deb11u1), libwbclient0:amd64 (2:4.13.5+dfsg-2, 2:4.13.13+dfsg-1~deb11u2), libsmbclient:amd64 (2:4.13.5+dfsg-2, 2:4.13.13+dfsg-1~deb11u2), python3-ldb:amd64 (2:2.2.0-3.1, 2:2.2.3-2~deb11u1), smbclient:amd64 (2:4.13.5+dfsg-2, 2:4.13.13+dfsg-1~deb11u2), samba-libs:amd64 (2:4.13.5+dfsg-2, 2:4.13.13+dfsg-1~deb11u2), samba-common:amd64 (2:4.13.5+dfsg-2, 2:4.13.13+dfsg-1~deb11u2)
End-Date: 2021-11-09 21:22:11
*********************************
Start-Date: 2021-12-02 07:17:25
Commandline: apt install libnss3
Upgrade: libnss3:amd64 (2:3.61-1, 2:3.61-1+deb11u1)
End-Date: 2021-12-02 07:17:26
Start-Date: 2021-12-29 09:06:22
Commandline: apt-get dist-upgrade
Install: libjs-qrcodejs:amd64 (1.20201119-pve1, automatic), swtpm-libs:amd64 (0.7.0~rc1+2, automatic), libposix-strptime-perl:amd64 (0.13-1+b7, automatic), swtpm-tools:amd64 (0.7.0~rc1+2, automatic), libopts25:amd64 (1:5.18.16-4, automatic), libzpool5linux:amd64 (2.1.1-pve3, automatic), swtpm:amd64 (0.7.0~rc1+2, automatic), libjson-glib-1.0-common:amd64 (1.6.2-1, automatic), pve-kernel-5.13.19-2-pve:amd64 (5.13.19-4, automatic), libtpms0:amd64 (0.9.0+1, automatic), gnutls-bin:amd64 (3.7.1-5, automatic), libunbound8:amd64 (1.13.1-1, automatic), libjson-glib-1.0-0:amd64 (1.6.2-1, automatic), pve-kernel-5.13:amd64 (7.1-5, automatic), pve-kernel-5.11.22-7-pve:amd64 (5.11.22-12, automatic), libgnutls-dane0:amd64 (3.7.1-5, automatic)
Upgrade: librados2:amd64 (16.2.6-pve2, 16.2.7), pve-docs:amd64 (7.0-5, 7.1-2), libcurl4:amd64 (7.74.0-1.3+b1, 7.74.0-1.3+deb11u1), ceph-fuse:amd64 (16.2.6-pve2, 16.2.7), libcurl3-gnutls:amd64 (7.74.0-1.3+b1, 7.74.0-1.3+deb11u1), proxmox-widget-toolkit:amd64 (3.3-6, 3.4-4), libpve-rs-perl:amd64 (0.2.3, 0.4.4), corosync:amd64 (3.1.5-pve1, 3.1.5-pve2), pve-firmware:amd64 (3.3-2, 3.3-3), ceph-mgr-modules-core:amd64 (16.2.6-pve2, 16.2.7), zfs-zed:amd64 (2.0.5-pve1, 2.1.1-pve3), chrony:amd64 (4.0-8, 4.0-8+deb11u1), zfs-initramfs:amd64 (2.0.5-pve1, 2.1.1-pve3), spl:amd64 (2.0.5-pve1, 2.1.1-pve3), pve-qemu-kvm:amd64 (6.0.0-4, 6.1.0-3), libnvpair3linux:amd64 (2.0.5-pve1, 2.1.1-pve3), ceph-base:amd64 (16.2.6-pve2, 16.2.7), libpve-cluster-api-perl:amd64 (7.0-3, 7.1-2), xxd:amd64 (2:8.2.2434-3, 2:8.2.2434-3+deb11u1), python3-ceph-common:amd64 (16.2.6-pve2, 16.2.7), librbd1:amd64 (16.2.6-pve2, 16.2.7), lxcfs:amd64 (4.0.8-pve2, 4.0.11-pve1), librgw2:amd64 (16.2.6-pve2, 16.2.7), libuutil3linux:amd64 (2.0.5-pve1, 2.1.1-pve3), libpve-storage-perl:amd64 (7.0-12, 7.0-15), ceph-common:amd64 (16.2.6-pve2, 16.2.7), vim-common:amd64 (2:8.2.2434-3, 2:8.2.2434-3+deb11u1), libpve-guest-common-perl:amd64 (4.0-2, 4.0-3), libvotequorum8:amd64 (3.1.5-pve1, 3.1.5-pve2), libquorum5:amd64 (3.1.5-pve1, 3.1.5-pve2), pve-cluster:amd64 (7.0-3, 7.1-2), wget:amd64 (1.21-1+b1, 1.21-1+deb11u1), proxmox-ve:amd64 (7.0-2, 7.1-1), lxc-pve:amd64 (4.0.9-4, 4.0.11-1), libcmap4:amd64 (3.1.5-pve1, 3.1.5-pve2), ceph-mds:amd64 (16.2.6-pve2, 16.2.7), ceph-mgr:amd64 (16.2.6-pve2, 16.2.7), ceph-mon:amd64 (16.2.6-pve2, 16.2.7), ceph-osd:amd64 (16.2.6-pve2, 16.2.7), proxmox-backup-file-restore:amd64 (2.0.11-1, 2.1.2-1), python3-cephfs:amd64 (16.2.6-pve2, 16.2.7), libcfg7:amd64 (3.1.5-pve1, 3.1.5-pve2), libcephfs2:amd64 (16.2.6-pve2, 16.2.7), qemu-server:amd64 (7.0-14, 7.1-4), libpve-access-control:amd64 (7.0-5, 7.1-5), pve-container:amd64 (4.1-1, 4.1-3), libcpg4:amd64 (3.1.5-pve1, 3.1.5-pve2), vim-tiny:amd64 (2:8.2.2434-3, 2:8.2.2434-3+deb11u1), pve-i18n:amd64 (2.5-1, 2.6-2), base-files:amd64 (11.1+deb11u1, 11.1+deb11u2), libradosstriper1:amd64 (16.2.6-pve2, 16.2.7), proxmox-backup-client:amd64 (2.0.11-1, 2.1.2-1), libgmp10:amd64 (2:6.2.1+dfsg-1, 2:6.2.1+dfsg-1+deb11u1), distro-info-data:amd64 (0.51, 0.51+deb11u1), proxmox-mini-journalreader:amd64 (1.2-1, 1.3-1), python3-rbd:amd64 (16.2.6-pve2, 16.2.7), python3-rgw:amd64 (16.2.6-pve2, 16.2.7), libseccomp2:amd64 (2.5.1-1, 2.5.1-1+deb11u1), libpve-http-server-perl:amd64 (4.0-3, 4.0-4), pve-manager:amd64 (7.0-11, 7.1-8), libpve-common-perl:amd64 (7.0-10, 7.0-14), ceph:amd64 (16.2.6-pve2, 16.2.7), libjaeger:amd64 (16.2.6-pve2, 16.2.7), pve-kernel-5.11:amd64 (7.0-8, 7.0-10), libzfs4linux:amd64 (2.0.5-pve1, 2.1.1-pve3), curl:amd64 (7.74.0-1.3+b1, 7.74.0-1.3+deb11u1), pve-firewall:amd64 (4.2-4, 4.2-5), libcorosync-common4:amd64 (3.1.5-pve1, 3.1.5-pve2), libnozzle1:amd64 (1.22-pve1, 1.22-pve2), python3-ceph-argparse:amd64 (16.2.6-pve2, 16.2.7), libknet1:amd64 (1.22-pve1, 1.22-pve2), pve-edk2-firmware:amd64 (3.20200531-1, 3.20210831-2), pve-kernel-helper:amd64 (7.1-2, 7.1-6), zfsutils-linux:amd64 (2.0.5-pve1, 2.1.1-pve3), libpve-cluster-perl:amd64 (7.0-3, 7.1-2), python3-rados:amd64 (16.2.6-pve2, 16.2.7)
End-Date: 2021-12-29 09:09:09
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After these... start problems ....