Hi Pieter,
I had a look in our logs and it appears that when your sandbox app calls our permissions API to check which roles the OpenID you posted has in your app, it does so with invalid credentials. This should be automatically fixed by redeploying to your sandbox, so could you try to see if this indeed fixes your issue?
I had a look at your sandbox, and was able to determine from the logs when the issue started. It turns out the issue which caused the "Woops" message was specific to the node to which you deployed, and has since been deployed. However it appears that the automatically generated credentials which your application uses to authenticate itself with our servers were not set correctly.
Can you confirm that you redeployed to your sandbox during the past 2 days? If not, doing so should fix it. If you did, can you please submit a ticket in our Support Portal (support.mendix.com) with as many details as possible about your issue, so we can fix it?