Following below steps should do the trick. I guess you already took the first step.

1. Delete the MendixSSO module in the Appstore module overview
2. Go Settings underneath the project "module"
3. open the tab RunTime
4. Remove  the After startup microflow: Select, None
   
5. Go to the appstore
6. (re)Download the module Atlas_UI_Resources
   
7. Open project security
8. Tab Demo users
9. Set selected Entity for demo account to Administration Account
   

You deleted the Entity ‘MendixSSOUser in the module MendixSSO that is still used. Either create it again or delete the usage of the entity  in the Settings, accessible when double-clicking on the error.

Good luck!

Here is the current status:

Any solutions for this yet that are known to work?