Yep I have this exact same issue too. Windows 10, HTC Vive Pro, RTX 2080 Super.
Before v9.0, DeoVR played most videos out of the box. Some videos failed playback with an error suggesting to install the LAV Filters and a link to https://github.com/Nevcairiel/LAVFilters/releases . After installing these filters, and maybe toggling various combinations of the "Direct Show" and "Alternative if default not working" settings during playback, it would play all videos I ever attempted, and everything was great.
Since v9.0, it consistently fails to open those same problematic files, with the same error as before: a box saying the video couldn't be played and to try installing the VP9 codecs / missing LAV filter. However, this box cannot be interacted with in any way, none of the buttons are functional and the only way to continue is to force quit the app. Because of this, it's not possible to change the video-specific settings during playback, meaning the player just cannot play these videos.
I have temporarily worked around this problem by downloading and running a pre-v9.0 version of DeoVR, as mentioned in this Steam Community thread: https://steamcommunity.com/app/837380/discussions/0/3151934575294138631/ . I tried all of the following things and nothing fixed the issue for the post-v9 version:
- changing the settings for all videos from DeoVR's main settings menu;
- re-installing the LAV filters and the DeoVR app;
- installing the K-lite codec pack.
It's not clear if the devs are aware of this problem. The fix might be as simple as allowing that error box to be closed and allowing configuration of the settings again during playback. Even if it is more complex than that, this is quite a critical issue affecting multiple people, for myself it affects roughly 40% of my video catalog, so I hope it is addressed soon.