You can try first if there is a problem on your end. Start by clearing the cache and removing any cookies from your site with the error.
Then check if you can log in again.
The error 500 means that something generally is wrong. So try to reload your last change page. If there is a error during the load start with a empty page and copy one by one the widget from the error web page to the new web page.
After each paste action update the new web page and reload the new web page to find the widget which gives the load error.
Be awere that you doesn't use a link that opens a web page in a new tab without a rel="noopener noreferrer"
Without the Rel you canget allkind of strange behaviour.