The MCP 2.6.5 update resolved the previous fatal error issue with the 2.6.4 build but contained a bug that prevented MCP from properly authorizing with your Mediavine Dashboard, which resulted in the inability to add new Video and Ad Settings blocks into your posts and pages. Existing Video and Ad Settings blocks are still functional, but the ability to edit them and add new Video and Ad Settings Blocks is not available in this version.
The root cause of this issue has been identified, and we are working on testing and QA for the 2.6.6. release now, which should go out shortly.
We will update again once that has been released.