Just click the sync icon and you'll see a wizard that helps you correct the problem.

I have experienced the same problem. I accidentally pasted a paragraph of text into the name field and Zotero synced it before I could edit it. I have since removed the long name, and when I access my library through the website the entry has the short name, but Zotero continues to give me the error message and won't sync, nor will it bring up the sync reconciliation wizard. I thought I might solve the problem by accessing my library via the website and deleting the offending record, but that function is not available. Any help? It hasn't synced in a week now.

Thanks, I didn't know I had to empty the trash, nor that it would sync the trash. That solved the problem.