This error message can appear for a number of reasons but one possible cause is a significant time skew between workload cluster nodes and all management cluster nodes.
This "unexpected validation error" is encountered in one specific case: the API server believes that the token is not valid yet. This will only occur when nodes have a significant clock skew that causes tokens issued from one node to be considered not valid yet by another node.
Many instances showing below error:
1398:E1026 13:40:56.079238 1 authentication.go:63] "Unable to authenticate the request" err="[invalid bearer token, Token could not be validated.]" and 52:E1026 12:48:11.627682 1 claims.go:126] unexpected validation error: *errors.errorString