The 500 error is due to nature hard to pinpoint as it can happen with/due to a lot of circumstances, it might be faulty code, htaccess issues, server error and so on.
Best advice is to look into the hosting error log. This should give one a likely clue what is going on, and where or what happens.
Curious : is it the older Event calendar plugin from v1? If so code internal could be the issue.