I'm having the same issue.
I'm still in the process of troubleshooting, but currently I'm not having the issue. I had just updated FramedBlocks, XaeroPlus, Puzzles Lib [Forge & Fabric], Structure Essentials, and Server Performance - Smooth Chunk Save. After booting it up, I had the issue, I closed and reopened it, and then I got an error about FTBRanks (which had happened before). It basically said it required a more up to date version of Forge, but I had just updated forge before updating all the other mods, so FTBRanks should have given the error when I booted it up before.
Currently, the game is running fine and my screen hasn't frozen and flashed.
I'll update this if more information comes up.
