New construction 2014: Problems with high humidity in the basement

Hello,
hereby I would like to describe my problem to the experts in the forum and hope I get a solution from someone that finally helps.
I have already spent weeks reading online and simply do not know what to do now.
From 2013 to the end of 2014, I built my own home with a lot of personal effort. Except for the basement, everything is great!
The house is fully basemented. The basement is also almost filled up to lawn/paving height. On two sides of the house, the basement still protrudes about 10-20 cm. The rest will be filled up by August at the latest.
Key data:
The basement has a gross area of about 100 m2
Divided into several rooms. Three rooms also have 1 x 1 m windows.
A straight staircase then leads up to the ground floor in a small passage room (about 6m2 in size). The staircase to the basement, however, is not separated again by a door since it is only a small room up there. In this room, there is a door to the garage and a door to the hallway on the ground floor. The passage room usually has a temperature of 20-24 degrees.
Floor structure in the basement:
Tiles
Screed about 5 cm
PE foil
Bitumen membranes (floor slab fully sealed)
24 cm floor slab
10 cm gravel layer
The exterior walls of the basement are built with 36.5 cm Poroton bricks. On the outside, first plastered and then covered with bitumen thick coating and 5 cm perimeter insulation in front of it.
No insulation was installed under the floor slab, and the exterior walls were also not insulated thicker than 5 cm. Since it should be a cold utility basement.
Now the big problem:
All exterior walls are currently getting mold spots in summer (but I already had problems in spring too!). The rooms also smell musty.
Most mold spots occur at the bottom wall area near the floor (probably the coldest spot). But also higher up, one or the other spot appears. I always try to combat this with methylated spirits or similar.
I have also reviewed all forums regarding ventilation behavior or similar. The humidity is always between 75-80%. If I use a dehumidifier, it only goes down briefly. After a few hours, the peak value is reached again.
I know that ventilating does not help in the long run and using a device continuously is also pointless. Then I would need one in every room and they would always have to run. How to ventilate, etc... I have already tried everything for weeks.
It only causes work and does not help at all!
Moisture from outside through the walls or floor slab I can definitely exclude!!! Residual construction moisture cannot be the cause either!!! I have already used several professional dehumidifiers where I removed water by buckets.
Somehow moisture keeps getting into the basement and causes everything to rot.
It also does not work with later installed heaters. It does get slightly warmer in the rooms, but it is still just as humid.
Currently, the basement temperature is about 16-18 degrees.
How can I now achieve a low humidity? Do the exterior walls have to be insulated from the inside afterwards to make it warmer and prevent moisture from settling? Will this also automatically reduce humidity if insulation is attached to the walls?
Would it perhaps work if I installed an insulation board all around at the bottom of the exterior walls in the floor area? So from the floor up 50 cm. How thick should the board be?
Unfortunately, I can no longer afford to excavate the basement from the outside and insulate completely again (especially thicker). This would not be cheap! Besides, the floor slab would still not be insulated from below.
Unfortunately, I did not know before construction started that cold (only minimally insulated) basements could cause so many problems! I cannot store anything in the basement.
I hope someone can help me!!!
THANK YOU VERY MUCH!

Moisture from outside through the walls or floor slab can definitely be ruled out!!! It also can't be residual construction moisture!!!

Following the logic, only the living spaces would remain as the source of moisture entry. I consider that less likely.

Has an expert already looked at it and identified the source? That would probably be advisable now to prevent consequential damage, since you are unable to remedy it.

Hello, thank you for the quick response. We installed 12 cm of insulation in the ground floor ceiling. Above that is the underfloor heating. This is for your information. I have not yet commissioned an expert. The gentleman from whom I borrowed the building moisture devices looked at everything and says it will eventually go away... but I do not agree!! It keeps coming back.

It keeps happening.

It comes from "somewhere". If the damage-causing error itself cannot be found, although I would possibly not directly count the borrower of the dehumidifiers among the experts, then the next step would really be to tackle the problem professionally.

This is important also to ensure that any warranty claims can be asserted directly and that the warranty obligor cannot excuse themselves by saying that the actual damage only arose because of professional inactivity.

The investigation does not initially cost much money, the possibly subsequent measure significantly more if it should be a construction defect. Delays in damage repair are likely to increase the costs.

I built my own home with a lot of personal work from 2013 until the end of 2014.

Accompanied by a building surveyor?

Floor construction in the basement: Tiles Screed about 5 cm PE foil Bitumen membranes (floor slab completely welded) 24 cm floor slab 10 cm gravel layer

Is there a soil report? I consider the 10 cm to be far too little, even with favorable soil conditions; which concrete was used for the floor slab? Was a foil additionally installed above the first row of blocks?

Unfortunately, I did not know before the start of construction that cold (only minimally insulated) basements can cause so many problems!

Usable basements, built correctly, do not cause problems! Rhine regards

Hello building expert,
we did not have a building surveyor with us. Our architect planned the construction of the basement this way and we carried it out ourselves.
The entire shell construction was done in-house (with trained bricklayers).
The floor slab is made of waterproof concrete. But I don't know exactly which type right now. The floor slab was also sealed from above with bitumen sheets.
Another foil is inserted in the first row of bricks!
Would internal insulation possibly help to avoid cold exterior walls?