NFS don't work anymore

Since today NFS isn't working anymore.

I already tried the following things:

1. Deactivate NFS, activate NFS

2. Reboot

3. Deactivate NFS, delete all NFS shares, create them new, activate NFS

4. Check via SSH:

systemctl status nfs-kernel-server
● nfs-server.service - NFS server and services
   Loaded: loaded (/lib/systemd/system/nfs-server.service; enabled; vendor preset: enabled)
   Active: active (exited) since Thu 2021-01-14 22:10:06 CET; 3min 46s ago
 Main PID: 4111 (code=exited, status=0/SUCCESS)
    Tasks: 0 (limit: 4684)
   Memory: 0B
   CGroup: /system.slice/nfs-server.service

I created the shares client ip xxx.xxx.xxx.0/24 and I can see the shares but I always get since today the message can't be mounted. (SMB for the same folders is working fine)

I have no idea why NFS stopped working from one day to another....

Can somebody help?

Ok made some research problem is something with nfs-common service:

● nfs-common.service
   Loaded: masked (Reason: Unit nfs-common.service is masked.)
   Active: inactive (dead)

unmask don't work, reinstall don't help

I found here a bug report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=934160

According to this it should help to add vers=4.1 to /etc/fstab, I tried but didn't helped in my case.