I just can't believe it, personally. Sure, things happen that are advantageous to mainstream mail clients from time to time; but, there are just so many non-mainstream mail clients out there (and the mail system is so well entrenched by this point anyway), that a change that has this drastic of an effect on clients would have people at Google's doors with pitchforks in mere moments. (Heck, you can already see the fire in people's eyes just from the folks here using Modest.) Honestly, if Google ever did cause something this significant, they'd be backing it out of their system in mere hours.

I'm fairly certain now that the first message Marmistrz sent me triggered the bug, even though I was able to download and read it. Every message after that one, from him or anyone else, generates the error message.

From the log messages, it appears that Modest has gone wonky about IMAP UID numbers. Essentially, it can still download mail from Gmail (or whoever), but some check at the end determines that the UID was wrong and gives up, even though it is apparent that both Modest and Google agreed on the UID up to that point. This really has to be a bug on Modest's part somewhere...