Lightning App: how to fix [Exception, DomainNotPropagated] error

This is an error or misconfiguration on Salesforce's side - at least it was in my case. You should open a support-case and refer to this link and maybe my case number (12670479).

They have resolved the issue and replied this

Hi, Hope you are doing well. This is to inform you that I was able to get in touch with Tier 3 to enable a permission so that Exception, DomainNotPropagated error can be fixed. Tier 3 has done this now and it is working as expected. Please inform if you need any other help. Thanks. Regards Developer Support Tier 2

Hard to say what that means exactly, but it works now...

Please refresh your page after 2 to 5 minutes. Error gets resolved. This error seems that recenlty you have defined your domain name.