Getting NFS to work on OpenWrt 21.02 rc4

I'm having some trouble getting my nfs server to work after a system upgrade from 19.07.6 to 21.02 rc4.
During the upgrade I chose to keep all configuration from 19.07, because I have quite some set-up, with MultiWAN, MiniDLNA, USB storage, Zerotier and lots of firewall configuratons.
In 19.07 I used NFS to access files on the attached USB hard drive, but for some reason the NFS server is giving me a headache in 21.02 rc4.
Everything else is working properly at the moment, the harddrive is recognized and auto-mounted.
I can browse the contents when ssh'ing into the router, but my machines are constantly failing to mount the nfs storage.
When issuing showmount -e, I can see that /mnt/sda1 is being exported, from either the router itself as from my pc
Can somebody help me getting it to work?

Have you any errors messages dump in dmeg ?
Which version of nfs server have you installed ?
Have you try to force nfs version protocol in mount command ?

Hello @erdoukki,

Sorry that I didn't provide logs straight away, but was not in the vicinity of the network in question.
I posted a dump of my dmesg, after a fresh reboot, it can be found over here at pastebin
Also I provide you with the System Log from within the luCi interface: System Log
What I find interesting to see is that in this log I see an error mentioned about unable to export the / partition due to unsupported filesystem.
Strange thing is that the root partition / isn't even part of my /etc/exports file.
The version of the NFS server package currently in use is: nfs-kernel-server 2.5.2-1.
Trying to force a mount using nfs version 3, shows a littlebit different behavior than using just plain nfs option

master has version 2.5.4. You could try that.

I think I ran into the same issue. I've reported it on GitHub.