Mold problem in new kitchen - how to prevent it in the future?

Have it checked why there is mold there, that cannot be determined reliably by remote diagnosis. Just as Dasch suggested.

Yes, of course a remote diagnosis is almost impossible, as with almost all problems. But it can probably be assumed with certainty that this is condensation water. Because now, after several days with the cabinet moved away, everything there is dry. Therefore, I am happy to accept tips and solutions on the condition that the theory of condensation water is correct - at my own risk.

As said: internal wall insulation with climate boards and then an autonomous refrigerator installed.

If you want to be sure: order an independent expert or call one of the well-known renovation companies.

If you want to botch it: attach clamp felt from the inside to the mentioned areas of the exterior walls (e.g., glue) and then reinstall the built-in cabinet including the refrigerator. Quite certainly, mold will still form behind it in the next few years.

By the way: a ventilation unit like Maicon will, in my opinion, not help at all.

Yes, this was just purchased. A new kitchen is out of the question – besides, as mentioned, there is no other possible location for a tall cabinet anyway.

In this case, however, I also consider it a planning mistake, since every kitchen builder would actually plan about 5 cm distance from the wall and finish it off with a strip at the front.

Possibly the least and more cost-effective evil would be to install the two cabinets next to it (below/above) in a narrower version, so that the refrigerator can be placed with the appropriate distance from the wall?

The problem is most likely (warning: remote diagnosis) caused by condensation (cooking, warm air) and the cold corner (exterior walls, "old" house).

Actually, there are only two reasonable options:

1. Complete exterior insulation (expensive)
2. Remove the interior plaster in the affected corner, glue special boards for interior insulation (see above -> "Klimaplatten") and then replaster.

Everything else will really be "botched work" and I fear it will mold again sooner or later.

P.S.: There are special paints that contain mold protection (e.g. from the brand Pufa). About 10 years ago, I patched up a corner like that in a rental apartment, and it worked well for 5 years. We moved out of there 5 years ago. (own house) Whether that works so extensively for you, I don't know.

Possibly the lesser and more cost-effective evil would be to install the two cabinets next to it (bottom/top) in a narrower version so that the refrigerator can be placed with the appropriate distance from the wall?

Unfortunately, that is not possible either, below there is a dishwasher next to it, next to that a waste system, and then the corner cabinet. All fixed widths. Maybe point 2. from Bernie will be implemented. Let's see what the expert says who will take a look at it at the end of the week. Thanks already for the responses.