I'm facing the same issue with the flatpak version of MakeMKV and the current and valid beta key:
redacted:~$ flatpak run --command=makemkvcon com.makemkv.MakeMKV reg T-PuNYycrTTHW4QEOfhDrMuJMpojNqmY5CGuESAKXzHvwBvBcXhtsJLj4g6OAXd1zfzk
Key not found or invalid
redacted:~$ flatpak run --command ...
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- Wed Aug 27, 2025 7:24 pm
- Forum: MakeMKV for Linux
- Topic: registering using makemkvcon
- Replies: 17
- Views: 49488