OK, this is a good start: so the issue is somehow TimeMachine related.
Now, I did a few tests with my NAS just now (a QNAP that supports TimeMachine).
The share (even though it is not mounted) appears in the list of TimeMachine disks I can choose.
Mounting a TimeMachine share seems unneeded, and TimeMachine mounts and unmounts the target share automatically (I'm on Catalina, and as far as I recall Mojave does the same thing). Running a backup and entering TimeMachine seems to work just fine without me even mounting the share.
I do see however that TimeMachine appears to be mounting/unmounting the share.
Now having said that: the mount/unmount of the TimeMachine share could very well conflict with CMN trying to mount a share.
Just for testing, I'd recommend removing the TimeMachine share from CMN completely, and maybe even close CMN for now.
Make sure the share is not mounted either - just in case it did get mounted before.
Now, tell TimeMachine to make a backup now and see what happens.
If it works any way like my setup, it should do a backup correctly.
After that, again without mounting the share (!), try to "Enter TimeMachine" (via TimeMachine menu option).
If all this works, we know that mounting a TimeMachine share is not required.
Next we can try running CMN (without the TimeMachine share of course) and see what happens.
You could leave the settings menu open en open up the "debug window" (reveal the "Show Debug Log" button by pressing CONTROL+OPTION+COMMAND or on a Windows keyboard: CTRL+ALT+WINDOWS). It may reveal something, or not ... 😋