This generally points to a problem with the WebDAV server, but if you provide a Debug ID for a sync attempt that produces the error I can give you more specifics.
Great, thanks. I have enabled debug and set it running again. It will take a while I have a stupidly massive library... 12,000 items 5gb+. Might that be it? I wonder what my server limit is...
Well, things seem to have resolved themselves. Last night I set it to resync with the Debug ID enabled but by the morning it was only a third of the way through and appeared to have stopped actually doing anyhting (I could still use zotero but the sync seemed to have stopped). I reset it and got on with my day. This evening I set up to give it another go and it all seemed to sync find, and very fast. Possibly because more than half of the data was already on the server but even then. Much faster than when I first set it up. It now seems to have completed fine!
Dan may be able to help more once you provide the debug ID.
Thanks any, fingers crossed, it's all sorted!