At least I found the problem, no problem with DavDroid at all!
I found the setting allowing Android to choose the best location for APP-installation.
SD-card was chosen, accounts are gone when restarting --> after moving the app to internal memory, everything worked fine 😉
Delete collection suggestion
I was testing the connection with the remote calendar server, and I tapped on the menu bar on CalDAV and selected “Refresh calendar list”.
After refreshing, the app displayed TWO instances of the same active calendar that I was using.
I long-tapped on a duplicate calendar and selected “Delete collection”.
Both duplicates on the app, and the ENTIRE (important and active) calendar on the server - got deleted!
And I have no backup data!
This option should come with a warning that says something like:
“WARNING! YOUR ENTIRE CALENDAR SITTING IN YOUR REMOTE SERVER WILL ALSO BE DELETED! DO YOU WANT TO PROCEED?”
Sorry that you lost your data. But DAVdroid is no back-up tool – it’s a management tool, and of course a user should be able to manage their collection (incl. a delete operation). There is a popup that warns you about just this:
Thank you for your reply.
It was a painful lesson.