With update to Thunderbird 68 an old default-Profile is not recognized. The solution is simple. Please open Konsole and type in the following command:
Thunderbird will now start with the Profile Manager where you can select your "old" default profile to start Thunderbird with.
On my maschine
randomstring.default was my original Profile, while
randomstring.default-release was the new default Thunderbird 68 created.