There are 9505 items in the library. It is an old database that came originally from Endnote and may have problems but always worked in Zotero.

The trash is empty.

I get a message that items must be the same to be merged

That's not quite what the message says—it says that items must be of the same type (e.g., book, journal article, etc.) to be merged.

9,500 items is large for an initial sync. It should be able to go through regardless, but if you have a large number of duplicate items you should remove as many as possible, empty the trash, and try to sync again.

If you still have trouble syncing, provide another Report ID and we'll try to figure out why it's failing.

Can you provide a Debug ID for a sync attempt that produces the error? Ideally don't do anything else in Zotero between when you start the sync and when you get the error.

You seem to be hitting a timeout or limit in some part of our infrastructure, though I'm still trying to figure out where. I've added a bit more logging on the server. Can you trigger another sync and post here when you do? No IDs necessary.

It may not fix it, but it should get you closer: do a Restore to Zotero Server from the Sync->Reset pane of the Zotero preferences. In the upload log on the server I see that there's some detritus left over in your database from all the items you deleted, and a Restore to Server will clear that out.

Try again, and let me know again if it fails.

(The issue that's currently preventing your sync is a number of overly long tags in your database. Zotero should automatically help you deal with those when the server returns that error, but for some reason—the size of your upload, and/or possibly something related to your network connection—your connection to the Zotero server is getting cut off before you get that error back. I'm still trying to figure out what's causing that, but you could actually go through your tags and delete any very long ones manually. There might be other issues, though, and dealing with those by hand is a pain, so we may need to continue to address the connection failure anyway.)

OK, I think I found the problem here. Give it another try. With any luck you'll see a dialog about long tags.

I looked at the tags and as you say some are very long; the ones I saw seemed to be from metadata I retrieved for PDF's - is that likely?

It's certainly possible. If you provide examples (in a separate thread) we can fix the translators to avoid saving those tags.

we would want to know how the tags got into your library. Presumably you didn't type them in by hand, which means they got automatically imported somehow.
If possible, we would like to prevent that by fixing the automatic import (aka translator), but for that we'd need to know where those tags came from.
So what you'd put in the new thread would be something like:
"When importing this record from www.weird-database.org to Zotero, I got a tag that was too long to sync."

my tags come from pubmed entries imported into an endnote database.
I imported the endnote data base into zotero without problems but they are too long for the server. it asks for the symbol to split, which is / but there are 4000 entries so manually pressing this for each one is not an option. Below is an example
Brain/*abnormalities/physiopathology Cerebral Ventricles/pathology Corpus Callosum/anatomy & histology Feasibility Studies Humans *Imaging, Three-Dimensional Infant, Newborn *Magnetic Resonance Imaging Schizophrenia/physiopathology