When trying to upload many files (>250) to a private bucket, the hf sync utility sometimes freezes completely. Is this a known issue?
250 files does not sound like enough to freeze sync by itself.
I would first check if it hangs before upload or during upload. Run the same sync once with --dry-run and once with --verbose. If dry-run is fine but the real upload stops, then it is probably not the file count. More likely one file, one path, or one transfer is getting stuck.
Also check if it always stops around the same file. If yes, that is the useful clue. If it stops at random places, I would split the folder and upload by smaller chunks just to narrow it down. Not pretty, but it usually tells you fast whether the problem is the CLI or some object in the folder.
Would also help to post the huggingface_hub version and the exact command. Otherwise this is hard to tell apart from a CLI bug, a network stall, or one weird file.