After debugging a lot I found that the problem is caused by the Add-on "Subject Manager 1.1.1". After disabling this plug-in sending works fine again.
-- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1797935 Title: Sending mail and saving drafts suddenly stopped working To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/thunderbird/+bug/1797935/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
