this is a really pesky bug. I’ve narrowed it down to IIS and most probably only people using this: https://codex.www.ads-software.com/User:ringmaster/IIS#Alternate_Rewrite_Methods
If you download the current dev version, I’ve made a temporary workaround, as I don’t know how long this could take to fix. Basically, you can enable permalinks, but Events Manager will still work with querystrings, e.g.
mysite.com/events/?event_id=x
Once the problem is sorted, you’ll be able to just revert to normal permalink slugs and the automatic 301 redirects should maintain your google rankings.
To enable this, you need to go to your config file and add this:
define('EM_DISABLE_PERMALINKS',true);
If you want, you can wait until the next update, I hope tomorrow but running a little late atm.