Oops... the system has encountered a problem (#102) - Retrying in 1s...The gmail support site seems to indicate that it's a known problem and the Gmail engineers are working on it, but it doesn't say much more than this.
Thankfully, I managed to figure out that it seems to be a conflict with one of my Greasemonkey userscripts: Force Wrap.
This userscript is meant to wrap text automatically. So often, I see pages that force horizontal scroll because the stylesheet doesn't take into account long blocks of text, and this hopefully fixes that.
After disabling Force Wrap, my Gmail problem has simply disappeared. Reenabling this userscript brings it back. Force Wrap doesn't apply to Gmail itself, but I can only guess it's messing with the Java somewhere, and hence causing this problem.
Anyway, for those having this Gmail problem, I just wanted to suggest looking at your Greasemonkey userscripts, as one or more of them may just be the culprit.