Our draft messages are disappearing!

If you create a draft message and leave "from" and "subject" fields blank, write some text, then save it and exit, the message will be saved in draft folder properly.

The issue occurs when you reopen this message from draft folder, modify it and save & exit again. The message will just disappear!

This weird behavior affects only Thunderbird (versions 24/31). Mac OS X Mail is ok.

Could someone help me, please?