I've tried to get the Oops view myself but all errors are directed to
the defined custom error template.
But I know that the app server has been going up and down a lot
because of manual restarts. Could this be the reason why our customer
get the Oops page instead of the custom template?
If ColdBox itself fails you will capture it within a Application.cfc onError() method. You should add some code to show your pretty error message from the Application.cfc onError method.
I added the onError method. In order to test it, I had to disable the
error handling in Coldbox (which I did by changing all catch(any) to
catch(database) in the coldbox.cfc).