Most probably this error was caused by the addition of an Open API to the access rights of a policy you’ve edited. As Open APIs don’t need to be set up with a policy, removing this API from your policy settings should resolve this issue.
Thanks for your reply. The API was configured with basic-auth (not an opened one). Anyhow, after few days of tests, adding, removing APIs and so on, I do not see errors on key request anymore…!