YouTube "Playback isn't supported on this device" after hard refresh

[DarkSky7]

Preliminary Checks

• I have read and understood the important section above.
• I have searched existing issues and avoided creating duplicates.
• I am not filing an enhancement request.
• I have checked that this issue cannot be reproduced on Mozilla Firefox.
• I have checked that this issue can be reproduced once I removed all my Mods and Custom CSS.

What happened?

• The page itself loads fine; only the video player is affected.
• Suspected area: player codec / EME (Widevine) re-initialization after a cache-bypass reload. The error state is not recovered by the player's own retry logic.
• May be related to the broader DRM/streaming issues class (see discussion Problems with viewing (some) streaming content #1831).
• DevTools on the affected page warns: "connected browser is more recent (154.0, buildID 20260818) than your Zen (1.21.15b)… may cause DevTools to fail" — probably only relevant to debugging, but noted for completeness.
• Workaround: a normal reload (F5/Ctrl+R) restores playback.

Evidence timeline

• Hard refresh → "Playback isn't supported" (extensions removed) → next hard refresh recovers.
• Hard refresh → broken → normal reload recovers.
• Normal reload → always plays.

Expected behavior

A hard refresh should behave like a normal reload for playback purposes — the video keeps playing.

Actual behavior

The player enters an error state after a no-cache reload and refuses to play until a subsequent reload (normal or repeated hard refresh).

Steps to reproduce

1. Open a YouTube watch page (e.g. https://www.youtube.com/watch?v=H6n3iNh4XLI). Video plays normally.
2. Press Ctrl+Shift+R (hard refresh / bypass cache).
3. The player shows "Playback isn't supported on this device" and the video will not play.
4. Press F5 / Ctrl+R (normal reload) → the video plays again.
5. Occasionally the broken state survives a normal reload and needs another refresh cycle; a second hard refresh sometimes also recovers it.

Screenshots and videos

No response

Version

Zen Browser 1.21.15b (buildID 20260818) — Firefox 154.0-based on Windows-11

What platform are you seeing the problem on?

Windows - x64

What component is this issue related to?

Other

Relevant log output if applicable

[Propheticus]

When the video does play, when right-clicking and enabling stats for nerds; what does the codecs line show? (e.g. for me it shows av01.0.08M.08)

I cannot reproduce on Linux, will try Windows later.

Suspected area: player codec / EME (Widevine) re-initialization after a cache-bypass reload.

YouTube does not use Widevine DRM, so that's an unlikely suspect area.
Codec could be it. Speculative: maybe it's a racing / timing thing where the page is already trying to serve you video in a codec your PC does not support before all the JS is loaded including retry and fallback logic. The next reload this is there and a fallback from an unsupported AV1 codec back to VP9 (or even H.264) does work.

Some things to try

• install AV1 video extension for Windows
• test in a new profile made in about:profiles (note your current default to switch back later) to rule out extensions or profile corruption