500 Internal Server Error out of nowhere

I’ve been successfully using RPi3 for a couple of months now until I run the latest software update yesterday. Haven’t changed anything else.

I can’t access GUI anymore as I’m getting:
" 500 Internal Server Error

Server got itself in trouble"

I’m a total newbie when it comes to linux so don’t even know where to start. I’ve been only able to export logs (from iOS app) but honestly they don’t mean anything to me.
Would appreciate any help - is there any way to fix this?

here’s my log file: https://pastebin.com/raw/QeikPZjs