Ok, fixed. There were 2 things wrong. First, for some reason the container port (should point to internal 80) was the same as the external port, so e.g. external port 6262 should point to container port 80, but pointed to internal 6262 as well. Could not change it in the docker port settings (greyed out, also after stopping the container), so i had to delete the 'container port' setting and add it again but now properly.
Now, the 502 error was gone, but wp showed an error 'something is wrong' and a white screen.
Some googling suggested to rename the plugin folder and try to start the site. That worked after that 1 enabled every plugin one by one until i found the culprint, some email form plugin. Deleted it. Site works fine again.