To sum it up:

1. It's some bug in our modest (according to fmg, in one of modest components), that got triggered by something (probably by mentioned change in folder management on gmail).

2. It isn't gmail fault, as other programs - even ancient ones - are still working fine.

3. It's not related to cssu, as it happens on vanilla ssu too.

4. Google sucks, anyway

// Edit

Someone wanna fill bug to Nokia?