Hey Acrobata,
This probably is not be related, but there is something wrong with your theme. Metadata and link tags get added to the body instead of in the head. That is likely because the wp_head() hook is not in the head. This should be the last element before the closing </head> tag.
info: https://codex.wordpress.org/Function_Reference/wp_head
If you created the theme yourself, you can simply move this element, else contact the author, or use a different theme, because this is pretty fundamental, so there may be other things wrong with the theme. (my2ct)
I've never come across an internal server error that only happened on a couple of screens. So, it beats me, but I would suggest to see if it helps when you deactivate plugins that may impact this. Or you could take a backup and update WordPress manually.
I hope this helps. GL!