Hi Lars,
In general, a red cross in Mendix mean that the environment is not working properly. Often undesired behaviour of your studio comes from marketplace modules that were deleted without being manually removed from the UserLib directory. If that is the case, I would suggest you clean up de UserLib folder in order tot get more stability.
More details on this https://docs.mendix.com/developerportal/deploy/environments-details/
Hope you this will help,
Jan