I have HP Proliant ML350 Gen9 server. I did nothing other than rebooting to a different kernel version when this happened. I always keep old kernels installed just in case. I use Debian GNU/Linux and not Ubuntu, although fundamentally Debian GNU/Linux and Ubuntu are the same. Currently the server runs on Debian GNU/Linux Buster.
Kernel version 4.19-(...
I guess you can boot into the recovery console? Try running e2fsck (man page) on it, e.g.:
e2fsck -f -v -C 0 -t /dev/sda2
If it doesn't help, please check your GRUB command line.
Edit: the issue was actually caused by dubious default mount options:
I've recommended to reduce them down to
which has ...