Howdy,
I believe this to be a symptom of the Z-index bug we fixed in the 3.1 Pro update. Are you able to update the plugin itself? We have a nice tutorial to walk you through this process: https://theeventscalendar.com/manually-checking-for-updates-to-modern-tribe-plugins/
You will notice in the screenshots an “Important:” notice about backing up your database and files. That is a standard procedure when updating WordPress or the plugins. If you have not already backed up, I would definitely advise you to click on the “back up your database and files” link when you see it and follow the steps it outlines.
Please let me whether or not this helps you to resolve the issue. Thanks!
– Brook