This is most likely your problem.
TLP only supports latest tests-passed (ie right now 2.9.0.beta10)
I updated literally 2 days ago with no issue. (Update: just performed another update on another site just now and it’s fine too).
I’m afraid you are going to need to update to latest to get this working again. The problem currently is Discourse core is moving Ember versions very fast, so TLP has to do the same and keep up. It seems the modern TLP code no longer works on Discourse 2.8.8.
If you can find the breaking commit and wish to submit a PR, I’d accept a version pin: Introducing .discourse-compatibility: pinned plugin/theme versions for older Discourse versions