You can switch off plugins one by one to see if this problem will be gone and this way to find a problem. The issue can be also with a theme, try to change it temporarily to a default one to see if this is the case.

You can find the source of an error to make it more certain.
Find where your server has an error log and look there these 500 errors, they will be label like ‘Fatal’ errors. Sometimes you need to switch on this logs on your server firstly, how – depends on a hosting provider.

After it will be clearer what to do and whom to contact.