Last week I had some trouble with my Gmail. I couldn’t send emails. Every time I tried, Gmail popped up a message Oops... a server error occurred and your email was not sent. (#014).
Searching for the meaning of this error (since error messages usually don’t make any sense) I found that it means the email was too long or something like that. Which simply sounds absurd, since the messages were only a few phrases without any attachment.
The end of the story is that I discovered the “Gmail Offline” feature was causing the problem. For some reason Gmail wasn’t able to re-synchronize with my PC, and this was causing the problem.
Bottom line, in my case, the solution was to disable “Gmail Offline”, ask it to erase all local messages and start over.
If you are having the same trouble, I advise you to just try disabling “Gmail Offline” and see if that solves the problem.