It's clearly a MacOSX issue
Apple does have a tendency to modify some small things, just enough to break things 😞
This does happen with ConnectMeNow, but also with my other application RenameMyTVSeries.
It is very frustrating, and also a reason why I stopped testing on Beta's.
Beta's are the worst when it comes to these kind of things - I spends days making a workaround, to find out that I can undo all that work because a problem no longer exists on the final release.
Anyhoo .... Coming back to CMN and NFS ....
Yesterday I updated to Big Sur 11.2.2 and tried an NFS mount with "vers=4,nfc", and this worked with CMN (using "Custom path" and setting custom path to the absolute path in advanced settings of that share). Granted, I have hardly any experience with NFS and mostly rely on info from you guys (for which I'm grateful). So I do not know for sure if my NFS share (on a QNAP NAS) is anywhere comparable with the shares you guys have.
Odd that you need to create a directory first though, since CMN forces creation of directories (for "custom mount" only, when using the "default mount" macOS creates the directory in /Volumes).
I di think Apple assumes using "resvport" is a potential security risk, even though I'm not sure why.
I'm confused haha ... 🤯