Ich bleibe mal beim Englischen
It was your decision to ask the question on the English part of the forum.
Great. Your suggestion works, if I pass true as third paramter during SoftwareSerial declaration
The third parameter set to true inverts the logic. I wouldn't expect the IR transceiver to use an inverted logic.
If you have a scope or logic analyzer you might connect that to your transceiver while you send some characters with your neighbors USB counterpart. That way you get a picture how the bits should look like and you can with the software until your signal looks the same.