Hello @wheelq and welcome to the forum!
Is this an old account? There are apparently problems new accounts see, but some ideas for workarounds.
Mega.nz error when check connection
Your error message looks different though, but possibly some Mega user (I am not one) could help you out.
There have also been a couple of issues filed on the third-party library that Duplicati ships, to access Mega.
Unicode is not supported #96
however this sounds like it was fixed in library 1.6.3. If you have Duplicati 2.0.4.5, I think you already have it, however if you’re on the previous beta 2.0.3.3 you don’t. And it might not be this same problem in any case.
EDIT:
Update the MegaApiClient package to improve performance for the Mega backend #3589
which I just discovered, is aimed at performance, but will also bring you library 1.7.1 probably in next canary.