This link was very helpful, thanks!
This is what i replied in there in case that his may help you:
In my case, y had 4 problematic mails in my INBOX, neither of them was the last one.
So this is what i had to do:
1- IN GMAIL: Get into every label and ARCHIVE every mail in there (so i move them away of my INBOX)
2- IN THUNDERBIRD: Check that there isn't any problems inside any labels (aka folders). Check that the problem still persists in the INBOX.
3- IN GMAIL: Create 2 labels
4- IN GMAIL: Move (yes, move... INBOX must be empty after this step) every mail from INBOX to LABEL 1
5- IN THUNDERBIRD: Check that the INBOX is empty and the error message is gone. Now check that the error is now in LABEL 1.
6- IN GMAIL: Go to settings -- Change the maximum page size to 100 (so you can see 100 msgs per page. This will make things faster)
7- IN GMAIL: Get inside LABEL 1, hit select all so you can select the first 100 mails. Then Move them to LABEL 2.
8- IN THUNDERBIRD: Check if the LABEL 2 troughs the error message.
9-A- IN GMAIL: If you DON'T get the error message in LABEL 2, you can move (IN GMAIL) this 100 messages to INBOX and leave this label empty. Then repeat steps 7 & 8 till you get an error.
9-B- IN GMAIL: If you DO get the error message in LABEL 2, then you know that one of the corrupted mails is one of those 100. Get inside gmail and try opening everyone of this 100 conversations. If the mail is corrupted you'll get the "Oops… the system encountered a problem (#754) - Retrying in 2:34... Retry now". Simply delete that conversation and check if you still have the error msg in Thunderbird.
That's it, lemme know if i wasn't clear enough (english is not my main language).
Hope it helps.