Another necro post (sorry), but this thread helped me troubleshoot so here's what was happening to players on my server:
They got the same error, but it was because they were missing a dependency code mod and the server for some reason wasn't able to auto-download it. Said 0 mods were missing, but 1 in fact was.
Testing the pack out by starting a singleplayer world with the mods that the server was able to download ended up triggering that 1 remaining dependency to be downloaded, so I was able to figure out which mod was missing.
Not sure why the discrepancy between Multiplayer and Singleplayer identifying the correct mods, but putting this here in case it helps someone else!