Please search the forum or check the Admin Manual. This stuff is documented and well discussed already. 
See Troubleshooting: Internal server errors
It’s also possible the error is before Nextcloud (I believe it is in your case, based on the output). That would mean checking your web server error log. Since you didn’t fill out the support template we can’t even speculate where that may be in your environment. However it’s likely in a well known location if you check your web server’s / OS distribution’s documentation (i.e. /var/log)
P.S. The support template you appear to have deleted when you posted, has some of the probable log paths in it.