Had the same issue; my issue was not a clogged pipe. The solution was to raise the ring seals slightly higher before installing the tank. This way, there's a better chance they will actually seal; that's how I solved my problem.
[ring seal = the round black plastic doing the jointure between the waste water tank and the base. There are 2 for the wastewater tank, one for the clean water tank.]

Hello, we have the same problem. According to our investigations, it's the seal on the dirty water tank that's not working properly and prevents depressurisation when the dreame empties the dirty water. For the moment, we're drying the dirty water tank lid after emptying it, and we've cobbled together a system that presses down on the tank latch so that the seal is correctly positioned. We'll mention this when we get back to you.

We have the same problem! I am now proceeding according to this forum and otherwise I will contact the manufacturer's customer service or the retailer.
https://de.forum.dreametech.com/forum.php