We are also facing exact same issue since 12 noon today. Everything was working fine prior to that and suddenly template based messages started failing with (#135000) Generic user error, only service messages as well as test_message template is working.

Yes, only the default template is working; the rest show a generic user error. This seems to be a Meta issue, and it hasn’t been resolved. I raised a complaint with direct support, but they said it’s a technical issue and directed me to the developer community. Unfortunately, no one has been able to solve the error there. It’s very complicated to deal with.