Ok, I could get it back working following the recommendations in this thread: https://theeventscalendar.com/support/forums/topic/just-purchased-license-and-entered-license-but-import-options-dont-show/
Still, I think you guys should have a look into this, a license key should not display such strange spontanious behaviour. I leave this issue open so you get to see it, once you read this, you can close it.