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 --ProfileManager
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.