Can't create new events

Hi! I have a problem with JEvents v3.6.93 on Joomla v5.3.3 on the website Titlehttp://www.alphenskozakkenkoor.nl. I am not able to create any new events. I am able to see the overview of existing events. When I either want to add a new event or edit an existing event I get an almost blank screen with the message: This page is not working HTTP error 500

This error also occurs when I want to add a new calendar or edit an existing calendar.

For the record: JEvents is working perfectly normal in the frontend, except also the editing/adding of new events. I can perfectly view existing events in the front end.

I am aware that I am using outdated versions of both JEvents and Joomla. I have tried to update JEvents to the newest version but then my whole site breaks down. I had to put back the backup to get it working again.

I have also tried to update JEvents to version 3.6.94. Then I get the error message: Call to undefined method Joomla\Filesystem\File::exists()
After that I'm also not able to open the component and the frontend of the website gives the error: 500 - Whoops, looks like something went wrong

My guess is that there has happened something with one of the files or the database.
It is possible to manually update to JEvents v3.6.94?
Does anyone have suggestion how to fix this problem?