Damp corner - What is the reason?

Hello dear community,

we have had a problem with a damp corner in our new building for years. The developer has so far claimed that it is due to incorrect ventilation behavior etc., i.e. wrong usage.
So about 14 days ago, when the big thaw occurred, the entire corner was damp again. However, we do not know exactly if that was the cause.
So far, the base of the house (transition to the ETICS) has been completely sealed again (except for a small spot behind the downpipe).
Since the entire corner was damp 14 days ago, we have heated the corner with a panel radiator and completely dried the room with a dehumidifier. We removed the gypsum plaster from the corner in order to check the brick directly and also to avoid or interrupt this sponge effect. The gypsum-containing plaster absorbs water like a large sponge, I wanted to interrupt that. Furthermore, we have inserted a moisture meter into the screed joint so that we can see what is going on there. Conclusion: After drying the room, the corner is still damp (99% humidity in the screed joint, after 3 hours of panel radiator 88%, but then rises again to 99%).

What could be the reason and how can we stop it?

(The downpipe has meanwhile been laid and connected to the drainage because standing water had also formed in front of the house)

Many thanks!

the base (transition to ETICS) sealed again completely (except for a small spot behind the downpipe).

Without really knowing much:
Wouldn't that cause all the water standing in front of the sealing to penetrate at that spot?
Do you really have standing water at the wall? No drainage?

If the downpipe was also happily splashing there, I'm actually not surprised. When was it connected?

What does the area look like from the outside? Have you ever measured moisture there? How far does the area extend?

In case of doubt, nothing helps except digging out from the outside down to the foundations and insulating.

Was the drainage (green pipe wrapped with felt; visible on the outside in the picture) close to the base or was the downpipe temporarily connected here? Additionally, I would be interested in the type of sealing. Was mineral sealing done here and painted over, or why is there no (black-gray surface) visible here?

was the drainage (green pipe wrapped with felt; visible on the outside in the picture) close to the base or was the downpipe temporarily connected here? Additionally, I would be interested in the type of sealing. Was mineral sealing applied here and painted over or why is there no (black-gray surface) visible?

I also noticed that, where is the foil etc.?

Well, when I look at the year the house was built, there was no sealing.... so if you want to use the room properly, you should create one.