@arubislander
I now deleted all my old accounts by deleting the corresponding apps. I then checked the accounts folder, it was empty of all files. In the meantime I had received an update. After rebooting and after reinstalling my nextcloud app, I could manage my accounts again. All working now.
Thank you once again for your assistance.

@arubislander
Yes, I had something similar in mind. There may be some corpse from an older installtion that prevents me from making any changes. I will look there first. Thank you.

@TotalSonic
It seems that I can't open an account there either. I click on the icon and it seems to be frozen. There might be a problem with my installation. I'm on the dev channel. Thank you for your interest.
Regards Martin

Unfortunately UBsync is not working on my device: bq E5 Hd. It does not allow me to create an account.

Happy Birthday Marius. Nice Show last night. Keep it up.

@gimenez
It looks great in my opinion. It does give a good oversight of all major features and subjects. It is easy to understand even for non geeks. Good job.

@Opolork
The layout of the indicators are one of the best design features of the ubuntu touch OS. All required settings in one single location. Loving it.

@Thatoo
What about "Ring" now called "Jamie" by "Savoir Faire Linux" https://jami.net ... it is open source and runs on all major OS's including Android and IOS. Since there it is also compiled for Ubuntu, it might not be too much of an effort to port it to UT.

@Ingo OK. That's good news.

A simple way to get rid of competition.