I think I have the same problem:
I have an image in the signature. When I try to send a mail out of Zimbra webmail in Internet Explorer (9) the send button get's deactivated and nothing happens. I only can close the mail, save it in drafts and send it from there - but the image is not beeing loaded then.
Sending the mail without the image is no problem. And the problem also doesn't appear when I send out of Firefox (6).
Does anyone know the reason or have a solution?
I attached a screenshot of my console-output but the quality is reduced while uploading so you can't read it very good... For the pros: The error appears in line 842 within the block:
ZCS NETWORK edition 7.1.2 (GA 3268) - Ubuntu 8 LTS 64bit