When i trying to post a youtube link tarting from a point in future... like t=13m05s
It does not show any thumbnail in the gallery homepage... just three big dots, i have tried to manually upload a thumbnail but it just does not show up... when i remove the time point, it stars showing up again...
I've just given this a go locally, and it works fine. Do you have any add-ons which either add an additional YouTube BB code media site or replace the default one? Specifically any which may support the time setting in the URL (which XF doesn't support by default).
Have you got a specific video URL that doesn't work?
There's also no reason uploading a thumbnail manually wouldn't work.
It's worth noting that I believe S9E replaces some BB code media site definitions, and may add new ones. Disabling or uninstalling the add-on does not actually return the existing definitions to normal.
I currently suspect that add-on to be at fault. I would guess if you added the video here, the thumbnail will work.
I fail to understand the wisdom of not allowing the video to start from a time stamp... this would allow us start a video from a timestamp which specifically stresses the point being made in the thread or media gallery... rather than watch the whole video...