Q: How to fix apt-get update error Mirror sync in progress?

Well, you may have actually hit it while the remote mirror was synching... in which case, waiting an hour or so, running apt-get clean (or apt clean) and then re-running the original apt-get update (or apt update).

If the error persists, you may want to notify the mirror provider if you can find a contact address, and then simply point your sources.list to a different mirror.

Helping a student university which was trying to run a simple apt-get update on an ubuntu docker container running on MacOS which was facing the same issue I ended up later finding that this issue was caused by "Screen Time", which is a MacOS Parental Control system, which apparently was blocking "certain" requests to the ubuntu repositories but not all.

Since the "firewall" had some restrictions for "adult sites", at some point when it was trying to fetch a hash from the repo, it would get a "blocked" HTML response from Screen Time on the host. This was especially interesting to me since I didn't think with the whole MacOS VM-based Docker subsystem, the HTTP networking within the container within the VM was subject to control by the host system :\