Wild guess: maybe something like this: Coursier resolution of Scala 3 dependencies - #3 by sageserpent-open is causing the problem?
That was down to me using an outdated version of Coursier, had to reinstall a brand new version to fix it.
Other than that, I presume you’ve already seen this: Proxy and mirrors | Metals or something similar as you’ve mentioned COURSIER_MIRRORS?
If you do find a solution, please share it back - Kinetic Merge is packaged as a Coursier application, and I’d like to know what to tell people if they are working in corporate firewall land.
EDIT: then again, there is: Coursier is unable to determine Scala version when cut off from the public Internet · Issue #2838 · coursier/coursier · GitHub , which also looks a bit like your problem…