I'm also running into this issue:
root@shv0:~# wc /proc/mounts
5679 34074 622502 /proc/mounts
I've tried working around this issue by updating the limit in ProcFSTools.pm based on the git commit in the bugzilla report:
return PVE::Tools::file_get_contents("/proc/mounts", 768*1024);
But...