I have been adding domains to apps without any issues for past couple months, following these instructions https://developers.google.com/appengine/articles/domains
Since past two days I have not been able to, when clicking Activate this service I get this error message "An error occurred while trying to install this application. Please try again later."
I have tried this also with older domains and with different apps and getting the same error, so the problem seems to be more widespread, not isolated to particular domains / apps.
Does anyone know how to solve?
This appears to be a problem on Google's end; it's not just you. I've filed a ticket with them for myself and am waiting to hear back. Your options are to:
Unfortunately, there doesn't appear to be anything we can do from our end to resolve the issue.