Permissions look like correct. Nothing modify... only K2 update...

I'm desperate. Planning a new install form zero but it is a very complex website in production with courses online and many technichal docs

I cannot reproduce the issue in any of my sites.
Is the error present in a vanilla install?

I don't know what vanilla instal is... so, the answer is no

New Joomla install with only K2.

I don't have K2 multiple images running. I don't really even know anything about it.

Try disabling it and see if that solves the issue.
K2 multiple images has been known to cause issues with K2 2.8.

Hello again!

Sorry for my long awaited reply, I've been out of the country.

As an update:

TL;DR
Disabling "System - K2MultiImages" managed to fix updating an item. I no longer receive a white page.

I updated Joomla to 3.8.1 and still have the same problem (Screen goes white). I checked my error logs as I did an update, it no logs were created.
I did disable the "System - K2MultiImages" and attempted an update, and I was able to make an update successfully without issues. However, with it being a system plugin I'm not so sure what that is going to affect.

I sent a message to the developer of the K2 Multi Images and we shal see how that goes.

Since this is a known issue, I don't think they 'll leave their users without an update.