Screen flashes black every other frame when packaging/launching my project + errors

A video to demonstrate the issue:

Also attached some errors. If I let the game run for a bit, one of these might pop-up and crash the game.
“Assertion failed: IsInRenderingThread()”
“A FRenderResource was deleted without being released first!”

This issue is in every level, but only when launching/packaging, for Windows. The build has the same issues on other people’s computers.

It happens both with the standard Unreal camera and my own camera class. I’m not using rendering textures or anything strange, like I said it happens in every level so far.

Ideas what might be causing it? Any console commands I can use to filter away certain rendering features to track it down?
I’m on UE4 4.18. A bit skeptical about upgrading since I’m a year into a project, but might test it if nothing else comes up.