Hello @harmione_johnsson ,
Yeah based on the reports you provided above, looks like it’s something with the incorrect ROOT MYSQL password related issue set in that “devkinsta_db” DB container - as you also tried to connect via mysql command line in that container / via its terminal but also getting access denied for that root user (which would also prevent DevKinsta to create/manage databases in that DB container).
Do you have any existing sites created in your DevKinsta already? or it’s just a brand new installation (and no local sites created yet)?
If you don’t have any local sites created yet in your DevKinsta , then you may want to exit and shutdown both DevKinsta and Docker Dekstop completely.
After that please remove and clean-up DevKinsta completely (remove/delete any folders and data related to it, for example, inside these folders: C:\Users\yourwindowsusername\AppData\Roaming\DevKinsta , and C:\Users\yourwindowsusername\DevKinsta ) .
Also please remove all containers, images, network volumes, etc. in your Docker Desktop.
Once they’re removed completely, please restart your computer, then you may want to re-install the Docker Desktop first, and once done, you can re-install DevKinsta (it will re-download/setup new images/containers from scratch).
The solution I suggested above has helped other user too in the other thread here. Hopefully it will be working the same on your end! 
Cheers,
Agus