Hi Hende, try a heated bed of at least 50C to start, with Kapton tape. Others have posted about this, so search the talk section for other recommendations as well (especially involving bed adhesion.)
I have The same printer and Love it. Had same problem before I did a bed leveling. Use the built in system to do a calibration. After that make print a square line 120x120mm 0.2 layer. If that gets even, try 0.1layer to 0.5 mm. Measure around the corners with calipers to check it’s 0.5. Otherwise try adjust very fine manually.
The finder can’t have a heated bed. And with PLA there is no need with proper bed leveling.
There is one awesome solution to solve the warping issue in any printer. For FF Finder, take out the build plate and invert it to expose the glass side. Put carpenter’s tape on it. Print a raft on top of it. Pause the printing. Put some quick glue on the corners of your print. Let it dry. Once it dries corners will adhere to the tape. Tape is already adhered to the smooth glass bottom. Now resume to print. It should print nicely. I tried and got 100 % results with 4 big prints of around 13cm X 13cm dimension.
I did almost what Abhishek suggested…I flipped the glass over and covered it with blue painters tape. I elected to try without using the glue and have had no issues so far. Just need to remember to change the tape once every couple of builds.