I have never seen this error before. It comes up when trying to run a test in Production. It happens when promoting the class (with just some capitalization changed, so it's not a real change) to Production. Class is running in API 49.0 and compiles fine in a full sandbox on Summer20 (49.0) and in a partial sandbox running Winter21 (50.0), and it fails in Production running Winter21 (50.0).
The error has no line number, and it shows when deploying using change set and via Gearset.
When trying to compile all classes (the link on the Apex class list) it gives one of those "provide this error code to Salesforce" pages. Not very helpful!
This started right after the org was updated to Winter21. Our nightly test run came back with an error. We hadn't edited the code at all.
Has anyone ever seen this error? A Google search for the error text yields nothing.