Flush-mounted shower tray - water is not draining - what to do?

My recommendation. With a Kärcher high-pressure cleaner and the accessory "Rohrreinigungsset PC 7.5 Length 7.75m" around €75, you can get the problem under control.

Remove the hair strainer and siphon bell, insert the high-pressure hose, turn on the device, and slowly feed the hose in cm by cm. It would be good if you open the cleaning cover downstairs in the cellar and collect the draining water using a piece of strong thick foil, etc., so you can see why the pipe was clogged. In a new building, you have to expect such blockages because paper plugs or construction debris can repeatedly get into the drain pipe either accidentally or deliberately.

Clorix or similar chlorine cleaners properly dissolve saponifications when dosed correctly. Hot water also helps and should be used regularly for prevention. Today's shower heads simply deliver too little water. The municipal utilities also have their fair share of trouble with that.

Be careful with chlorine only on colored items!!!! The color disappears faster than you can blink.

Do not forget or research beforehand what has been installed: what drainage capacity the shower channel has (manufacturer specification), how the siphon is constructed, and with which pipe diameter and slope the channel is connected to the drainage line. Sometimes only DN 50-40 and a meager slope of 0.5-1% remain to fit within the existing installation heights/connection heights. If necessary, a separate pipe vent is also required if the maximum connection length according to DIN to the downpipe is exceeded.

In this situation, I would generally be cautious with brutal pipe cleaning methods (high-pressure cleaning/flushing). Level-access showers are sealing and drainage-sensitive components. This is especially true for installation done by oneself. Pipe cleaning should preferably not cause a hidden leak.

Any chemicals attack the pipes; so keep your hands off.

The usual PVC pipes installed for decades are very resistant to acid chemicals!
If anything, it is the seals themselves, but they also last for years if they are not directly exposed to acid.

xxx.mcm-Systeme.de/Chemische-Bestaendigkeit-von-PVC-PE-und-PP