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Does someone have an explanation?
Yes, don't connect a DMM to the output
It's completely unsuitable for what you're trying to do, and the IR IC won't work properly unless you've got it connected correctly, they are quite fussy about their connections (decoupling and short connections). There are examples in my PIC IR tutorial that work fine, built on veroboard - breadboard probably isn't a good idea, assuming that's what you're doing.