Thank you, the bad news is that I dont have a clue as to what might be the problem.... :-/
As I have told you before this project is not funded, so the software components are not being updated. At some point bugs will occur, and that is probably what we see now with the mail/message component.
Looks like it is time to make a big decision regarding the future of this project...
If someone can make it work, then we can move the updated website (with kunena 5.X) from a test domain and back to the Roland-arranger.com domain.
Today I have contacted a professional developer who is a kunena / Joomla specialist. I will ask him what he charges to do the update.
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