Tracker.Start() is defined in internal/lps/progress/progress.go. Its fourth parameter is a protocol.ProgressToken. Although the Go type of ProgressToken is interface{}, the comments in tsprotocol.go (and in the LSP specification) say it must be either an integer or a string.

Most of the calls to Start() pass a nil value, which the code in Start() converts to a randomized string.

The proposed fix is to change the type of ProgressToken to string (which matches all its uses in gopls), replace the nil arguments with empty strings, and replace the test at line 89 of progress.go with a test for an empty string.