Wow! Thank you both @james and @fbartels! That was such quick help!
That was it. The manifest was missing tags; added them and regenerated CloudronVersions.json with the cloudron CLI (the custom script is retired). The raw URL now serves the fixed file, so it should pass on the next sync: https://raw.githubusercontent.com/OrcVole/windmill-cloudron/main/CloudronVersions.json
