Hi,
When i look in the log generated by a rule execution, i find the information about the send mail action:
That's a classic symptom of "workflow tables don't match issue tables". It's quite hard to get into that state without broken code/scripts, and your case sounds very odd - the only way I can imagine you'd be able to get there is to have two post-functions that are doing unusual things. One (not the post-function you've removed) is breaking the data. Then the post-function you removed and put back, is (accidentally?) fixing the damage done by the first.
Anyway, cause not withstanding, there are four things you can do. In order of preference that I'd do them:
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