why i cannot play and playback some videos on youtube at random time of the video?

[inukjuak90]

"Error 500" is a server/cookie issue.   Chances are YouTube is having some
"internal" connection issues with their servers.   However, one thing you can
definitely do is clear your Cache + Cookies + History, then shutdown/reboot.

Mozilla Firefox:
→  Tools  →  Options  →  Privacy ('key')  →  Clear Now
[x] Browsing History
[x] Cache
[x] Cookies
[x] Offline Website Data
[x] Authenticated Sessions
→  Clear Private Data Now

Google Chrome:
→  ≡  or wrench  → Tools  →  Clear browsing data
→  the beginning of time ▼
[x] Clear browsing history
[x] Empty the cache
[x] Delete cookies and other site and plug-in data
→  Clear browsing data

Don't forget to totally shutdown+reboot afterwards.

:-)