"100 Current database version is not set (). I can't update! I tried to update to database version 3.7.3_2017-08-04. Please contact the developer for help.If you want to try a fix yourself, please have a look at this page: https://www.svenbluege.de/joomla-event-gallery/event-gallery-manual/faq-sql-error-during-install. "
I don't think the FAQ page applies because I've never downgraded Event Gallery.