Game started randomly breaking and throwing errors after engine update


I revived one of my old projects which was made in 4.16. At this version it still works like a charm. I wanted to upgrade it to 4.21. But there it totally breaks and starts throwing errors at a random point while playing. Upgrading to 4.18 worked, 4.19 is the first version this error happens. Does anyone have a idea what could cause this and if something specific changed since 4.19 that could cause it? An image of the error log is in the attachment.

I already tried migrating all files into a freshly made project but same problem. And I have no clue how to further debug this since it happens randomly and used to work. Also everything is made in blueprint no c++ used.