It is only getting worse and worse and becoming pretty annoying.
I found a thread and thought I now could solve it with that clear explanation (although the error of the library still is a puzzle to me as a non-expert):
Then Thunderbird kept saying "Your Thunderbird profile cannot be loaded. It may be missing or inaccessible." and now it couldn't be started at all anymore. So now I have no mail anymore and can only mail via roundcube at the moment.
Then I removed Thunderbird in order to reinstall it starting from scratch.
But all I got now is Thunderbird with its previous settings and still ever the same message "Your Thunderbird profile cannot be loaded. It may be missing or inaccessible.
=> How could I make a clean start building it up from zero?
Is there by the way a similar facility like in Windows of "system restore" to a previous version?