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The 2-component bitumen waterproofing offered in your hardware store should not be used. It is only suitable for waterproofing from the wet side, not for negative waterproofing. In addition, bitumen waterproofing is not permanently UV-resistant.
The Erika waterproofing membrane is theoretically possible. BUT: It is definitely not suitable as a floor covering and cannot be tiled directly or similar. A screed must definitely be applied on top, and possibly even insulation for decoupling between them.
The suggested Remmers MB2K and other bitumen-free hybrid waterproofings can be used here. They cost a few euros but are definitely permanently watertight and firm.
Significantly cheaper would be, as suggested at the beginning, the flexible mineral sealing slurry. These are suitable for negative waterproofing on the interior side of the room, impact- and scratch-resistant, UV-stable, and can also be tiled directly. They also cost only about 1/4 of the hybrid waterproofing. Execution details and procedure I have already described in the first post. You can also get it in "your" hardware store as "MEM Dichtschlämme" for just over 1€/kg. Of course, similar products with comparable properties are also available from other manufacturers.