What it says in the title. Today I ran into this issue, and I’m attempting to diagnose the root cause.
It seems like it has something to do with one of the mirrors going down. Their registrar’s account suspension page incorrectly returns HTTP 200 instead of an error code, so pacman downloads the error page thinking it’s the Endeavour OS package DB, causing a corrupted DB.
I was getting 404s (presumably from the dead mirror) and ran eos-rankmirrors to try fixing the problem. If it doesn’t download the latest mirror list before ranking the mirrors, that would explain why it didn’t fix the problem. (And, combined with the seemingly incorrect HTTP code, caused a corrupted DB.)