Air tightness in the area of the pliers

Hello dear forum,
I am Mirco from Neumünster.
We have bought an old building which we are currently renovating on our own (with the help of some craftsmen, carpenters, heating installers, etc.).
Now a question arises for me...
I am currently in the process of connecting the vapor retarder.
That is working so far. The only thing that is puzzling me right now is the connection of the airtight layer in the area of the last or first rafter bracket.
There is a huge air gap there (see pictures) where I am unsure how to seal it.
I have already tried to spray with Premur, which ran out again overnight.
I have briefly thought about PU foam, but unfortunately that is not permitted as a vapor retarder / vapor barrier.

I would now be very grateful for some ideas for the implementation.

Best regards from the far north

Depending on the size of the gap, stick a narrow strip of vapor barrier foil or a wide corresponding tape along the beam and plaster it onto the gable wall. You also have to tape and plaster the transition from the slope to the gable wall btw. ...and since I'm at it: You should sand and/or prime such old beams before applying the tape.