Right now I do not know the root cause, but I think the issue your error is refering to is reported here.
Code:
Sep 27 10:18:36 Proxmox kernel: pci 0000:01:00.0: can't claim BAR 6 [mem 0xfff80000-0xffffffff pref]: no compatible bridge window
Sep 27 10:18:36 Proxmox kernel: pci 0000:01:00.0: BAR 6: assigned [mem 0xc1080000-0xc10fffff pref]
Sep 27 10:18:36 Proxmox kernel: pci 0000:00:01.0: PCI bridge to [bus 01-06]
Sep 27 10:18:36 Proxmox kernel: pci 0000:00:01.0: bridge window [io 0x4000-0x4fff]
Sep 27 10:18:36 Proxmox kernel: pci 0000:00:01.0: bridge window [mem 0xc0000000-0xc1ffffff]
Sep 27 10:18:36 Proxmox kernel: pci 0000:00:01.0: bridge window [mem 0x90000000-0xafffffff 64bit pref]
Sep 27 10:18:36 Proxmox kernel: pci 0000:00:1c.0: PCI bridge to [bus 07]
Sep 27 10:18:36 Proxmox kernel: pci 0000:00:1c.0: bridge window [io 0x3000-0x3fff]
Sep 27 10:18:36 Proxmox kernel: pci 0000:00:1c.0: bridge window [mem 0xc2500000-0xc25fffff]
Sep 27 10:18:36 Proxmox kernel: pci_bus 0000:00: resource 4 [io 0x0000-0x0cf7 window]
Sep 27 10:18:36 Proxmox kernel: pci_bus 0000:00: resource 5 [io 0x0d00-0xffff window]
Sep 27 10:18:36 Proxmox kernel: pci_bus 0000:00: resource 6 [mem 0x000a0000-0x000dffff window]
Sep 27 10:18:36 Proxmox kernel: pci_bus 0000:00: resource 7 [mem 0x8fa00000-0xfeafffff window]
Sep 27 10:18:36 Proxmox kernel: pci_bus 0000:01: resource 0 [io 0x4000-0x4fff]
Sep 27 10:18:36 Proxmox kernel: pci_bus 0000:01: resource 1 [mem 0xc0000000-0xc1ffffff]
Sep 27 10:18:36 Proxmox kernel: pci_bus 0000:01: resource 2 [mem 0x90000000-0xafffffff 64bit pref]
Sep 27 10:18:36 Proxmox kernel: pci_bus 0000:07: resource 0 [io 0x3000-0x3fff]
Sep 27 10:18:36 Proxmox kernel: pci_bus 0000:07: resource 1 [mem 0xc2500000-0xc25fffff]
Sep 27 10:18:36 Proxmox kernel: pci 0000:00:1a.0: quirk_usb_early_handoff+0x0/0x7c0 took 18752 usecs
Sep 27 10:18:36 Proxmox kernel: pci 0000:00:1d.0: quirk_usb_early_handoff+0x0/0x7c0 took 19511 usecs
Sep 27 10:18:36 Proxmox kernel: pci 0000:01:00.1: extending delay after power-on from D3hot to 20 msec
Sep 27 10:18:36 Proxmox kernel: pci 0000:01:00.1: D0 power state depends on 0000:01:00.0
Sep 27 10:18:36 Proxmox kernel: pci 0000:07:00.0: set MSI_INTX_DISABLE_BUG flag
I will have a deeper look to find out what caused this.