I have been in some kind of half in half out state with !OGEhHVWSdvArJzumhm:matrix.org, see #6520. Today I clicked clear cache in Riot and sync was failing with the following error:

Attempting to use the purge room API on !OGEhHVWSdvArJzumhm:matrix.org returned:

{
    "errcode": "M_NOT_FOUND",
    "error": "Unknown room !OGEhHVWSdvArJzumhm:matrix.org"
}

The only way I could figure out how to get my sync to work was to go into the Synapse manhole and run hs.get_datastore().purge_room('!OGEhHVWSdvArJzumhm:matrix.org') which let the sync complete.