Historically, the answer is often that they’d rather you use their client (IIRC – did you find such reports?).
Are you saying that the Mega client keeps on working, first time, and later (even after Duplicati is hung)?
If you’re on (or are willing to be on) GitHub, you could ask if MegaApiClient author has seen such things.
There weren’t a flood of complaints there, but this sounds at least a little similar to that one issue I saw.
The Process Explorer (easier after install) or
netstat -ao | findstr mega or maybe
findstr <PID> where the Duplicati PID is from Task Manager (the child is the one to get, or you could just test with both) might be interesting. It sounds like you probably got an ESTABLISHED connection that didn’t go as usual.
I’m pretty sure this is all encrypted, so it’s hard to see what’s being said without some special techniques such as network tracing, which I think can capture the program view of the data without the encryption…
Mega remote couldn’t login error is rclone’s version of a Mega file listing (lsf) failure with some comment.
Mega: randomly failed to login is a slightly older issue with more commentary. Not all topics are negative.
If you dare try your hand with rclone, and can get it to work, then Duplicati Rclone destination might work.