No, the change was made a while ago. Actually, I always install the updates when they are available. But I don't try all functions afterwards. Therefore I can't say since when the login error exists.
All 3rd party extensions have been removed. The last extension (deactivated until then) was "Auto Welcome". After I uninstalled it I only see a white page. No error message anymore, but also no user data... Slowly I am afraid to shoot down the forum completely...
This is the last mail from the support of the hoster:
Then the conversion to innodb should be done via PHPmyAdmin. Before you start your work, you should create a backup of the database, so that you can restore it to the current state in case of emergency.
Then a short tutorial on how to convert MyISAM to InnoDB.
Would that help?