Thank you @Sunil! My workaround was to move the mails of the inboxes of all accounts to an inbox-subfolder and to move them back from the subfolder to the inbox after the upgrade. But cool to have a patch 
Thank you @Sunil! My workaround was to move the mails of the inboxes of all accounts to an inbox-subfolder and to move them back from the subfolder to the inbox after the upgrade. But cool to have a patch