Error when using GroupShare 2019 translation memory: "An error has occurred whilst using the translation provider [TM name]: One or more errors occurred"

I'm having problems with the Groupshare server-based TM’s of a client based in Spain (I’m in Portugal) in my versions of Studio 2017 (Windows 7 desktop) and 2019 (Windows 10 laptop) located in different towns, but which are giving the exact same error.
I configured everything according to the client's instructions and the server appears available, but each time I confirm a segment with Ctrl+Enter, the green "tick" appears but an error message appears in the "Messages" tab.
Both my Studio versions are up to date and I even followed the instructions of the following link:
gateway.sdl.com/communityknowledge
The customer is using Groupshare 2019 and will shortly be migrating to 2020 and has already changed permissions to see if it solves the problem, but everything remains the same. I've removed and reconfigured the server several times and used the client’s TM’s in different language pairs, but the problem persists.
I can’t even access the Settings of those Groupshare server-based TM’s as the error message “one or more errors occurred” appears every time I click on “Settings”

Everything else is working fine with file-based TMs.

Screenshots:

6457.Groupshare error.docx

4314.Groupshare error 2.docx