API keys were first introduced in Brandmeister in 2018, as API v1. In August 2022, developers improved the API features and created API v2, which changed the keys format. API keys created after 2022/08/19 are in v2 format. Keys created prior to this date are in v1 format.
I did generate a new API key, and suspect it will work, but the OS3 reports it is invalid.
I suspect checking the new v2 keys for validity may not be functioning yet.
Here is a link to the Brandmeister communication on this topic: