For an estimated 20% of all YouTube videos I try to watch on one of my computers, I cannot get them to buffer at all. I will get the loading spinner, until about a minute later the player will say that "an error occured, please try again later."

I have ruled out a lot of possible causes and don't know where else to look.

* It's not a general YouTube or video playback problem. I can watch the majority of YouTube videos fine. It's certain ones that I can't get to play no matter how often I retry. I haven't had any problems with video playback on other sites, although I rarely watch video outside of YouTube.
* It's not a browser or extension problem. The videos that play fine, play in all browsers. Those that don't, won't load in any browser I have installed. In particular, I tried Firefox 32.0.3, Chrome 44.0.2403.130, and Opera 31.0.1889.99. I'm running Windows 7 Enterprise SP-1.
* It's not a Flash player problem. The affected videos won't load in either the Flash player or the HTML5 player.
* It's not an issue with a particular stream. None of the offered stream qualities will load, while for working videos, all of them load. (In particular, there's no difference between regular and DASH streams.)
* It's not an issue with particular videos themselves. The same videos work fine if I play them from any other device. Also, on some other day it might be different videos that will not play, while previously broken ones work again.
* It's not an ISP or connection issue. The computer in question is a corporate one, set up identically to other computers in the room which share the same connection. Other computers play all the videos fine without buffering problems. Mine seems to be the only one affected. There are no active software firewalls or local hosts files, and I'm using the same DNS as everybody else.
* It doesn't seem to be a cache issue. I cleared all local data of the Flash player and all browsers. The problem persists, and still always for the same videos.
* It's not a temporary issue, this has been going on for the 18 months I've used this computer.
* Incidentally, I can also not get to any of the streams by using stream downloader plug-ins. They will get 0 Bytes and just time out.

What could be other possible causes for this issue, or what should I try doing to narrow in on the problem? My current workaround is to RDP to a different computer and watch the problematic video there, which is not the most comfortable of solutions.

Example for a video currently playing fine: [Video 1](https://www.youtube.com/watch?v=UznHTBZIa8E)  

Example for a video currently not playing: [Video 2](https://www.youtube.com/watch?v=NL3BlDYWmRk)

**Some more observations:** I noticed that on the pages of those videos that won't play, also none of the thumbnails of related videos will load. The browser status bar shows that there is no response from `s.youtube.com` nor `s.ytimg.com`. As before, though, on pages of other videos, both the stream and the thumbnails are served fine from the same servers. After a few hours, the affected video pages will be different ones.