Modest is currently broken, as far as I'm concerned it's been broken for a month or so at one level or another. I hope they fix it soon.

EDIT: I should say that for gmail access the only reliable way I have is via the web browser. I have been using Modest for over a month now and it's ability to work and not crash has been spotty at best. At least it's better than the built in client that doesn't work at all.