I am getting following error while sending email to Emailservice

(Undelivered): 554 common.apex.runtime.ParseException  What would be the problem.

The code was running perfectly till last Friday 21-Mar-2015. I haven't change anything in code since then, but it's not working since two days; stange behaviour. Although even If I had any problem in my code, email should receive in Salesforce and then I can use debug log to see any code exception but I am receiving bounce back email with above error message.

Hello,
we are experiencing an code coverage problem on our production environment. Last week we deployed an change set without any issues and the code coverage was at 75%. 3 days later we tried to deploy an other change set, but this has failed do to code coverage error. The code coverage is now at 73%. The problem is, this change contains no code and is only an flow.
There were no changes to any code in those 3 days and no changes to validation rules or work-flows (checked via audit trail), all tests succeed, the only problem is in the coverage. We are generating our test data via factories and using seeAllData=false and aren't using any record in the org for testing.
The code coverage was at 75% at the time of the deployment of the first change set, it would not deploy otherwise. How is is possible, that it is only 73 now?
Classes recompiled, test history deleted, we get the 73% when deploying or running all tests(UI and IDE).

I have found this issue on the forums
https://success.salesforce.com/issues_view?id=a1p30000000T1m1AAC
and have checked the ApexCodeCoverageAggregate, but there is no problem.

I have even tried to validate an change set containing 600 lines of i++; (I know, ugly) and haven't got even 1% more coverage. We are currently using about 2M chars of code.

This is an blocker for us, any help would be appreciated.

Thanks.