For a few days, my offline Gmail was refusing to synch up with the server. Every time I would try, I would instantly get an ! in the synch status and it would say “Synchronization has stopped unexpectedly”. Well, yeah. I had figured that part out.
I’m sure there are lots of reasons for this, but you can open the troubleshooting page by going to Settings -> Offline and scrolling down to the link for the Troubleshooting Page.
On my page, I noticed several errors for Failed to get blob down at the bottom of the page under “Recent Errors”.
A BLOB, of course, is a Binary Large Object. I realized at that point that I had unsynchronized messages with images attached (that is to say with BLOBs attached).
I tried moving these to Drafts, but that still didn’t work, so I copied and pasted the contents into a text editor for safe keeping, deleted the messages, cleared the browser cache, closed the browser, reopened, and reloaded GMail.
I’m sure there are a lot of other things that can cause this, but I didn’t find any good information on this on Google Groups or with a quick search, so hopefully this will help someone or at least give some directions to look in.