Condensation and mold on the supply air duct - What to do?

Thank you for all the answers.

I will check the tightness of the wall penetration over the weekend and seal it with silicone from the inside and outside if necessary.
I had already sealed the frame once with silicone because I thought there was a draft there. But it did not help.
I will also check whether the frame fits snugly or if there is a cavity.

Oh, one more thing: how do I best clean up the mess? Sagrotan mold remover from DM?

The most effective mold removers are those containing chlorine. It is important to let them work for a long time and to repeat the application. If you can't get something like that, then mellerud is also not bad; also repeat the application even if nothing is visible anymore. The mold remover from sSagrotan is also not bad if you can't get chlorine-containing ones.

For sealing, use PU foam if possible

So, a bit late my feedback:
It is dry!!!
even at freezing temperatures.
It was definitely due to the small air gap of the outer pipe.
Thank you very much for your tips!

Hello,
the decisive factor is that the insulated pipe is routed completely through the wall and then ends in the panel. This penetration should also be perfectly sealed. Pipe to wall.