I wasn't able to reproduce the problem using jira 6.3.15, I'd like to ask you further details to help me find the problem: Which browser and which version are you using? (Chrome 32, Firefox 21, Safari 4, ...) Which language do you have configured in JIRA? (English, German, French, ...) Could you provide screenshots of the error in the incoming mail screen and also screenshots of the plugin configuration?
After talking with Vincent Briere via email, we were able to find the problem.
Anyone having this problem please check the following:
Screenshot of field:
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