I just copied my project folder to do some trial and error testing.
After seeing the below copied message in the latest log (along with other error lines), I just tried my luck and deleted the CachedAssetRegistry.bin file in the Intermediate folder of the project.
So with this file deleted, I can finally start the project again.
[2021.06.18-14.11.13:057][ 0]LogThreadingWindows: Error: Runnable thread FAssetDataGatherer crashed.
Unfortunately, when wanting to open the map I worked on the last time before this issue happened, I get this message “Failed to open map file. This is most likely because the map was saved with a newer version of the engine.”
Luckily I still have a slightly older copy of this map which I can open.
But maybe someone knows if there’s a way to restore the proper map file even after deleting the CachedAssetRegistry.bin?