I was able to reproduce this (kind of) problem on one video, others work fine. Since I actually received a different error message than you did (mine said 'not a movie file'), can you post a few YT vidos affected by your problem so we can have a look at what they have in common?
The videos you linked to all work for me (see screenshot).
Which version of the QTE do you have (should be: 120)? Do you click "open SD" or "HD"? Which version of the browser? Which Mac OS X version? Which QuickTime version? Do you have Perian installed or other plugin codecs? There are so many variables…
OK, so my setup is: 10.5.8, QTE 120 (I use SD), TFF FPR 13, QuickTime 7.7, but QuickTime Player 7.5.5*, Perian 1.2.3, several legacy codec plugins such as Xvid, DivX, WMV, MPEG2 etc. I think Perian is most likely what makes the difference.
*all later versions have a QT Pro export crash bug in the player that Apple never fixed for PPC.
Brian J. Patter...
on 13 Mar, 2019 12:35 AM
The issue actually ended up being "cache poisoning." I flushed the cache in case "something" was left behind giving QTE indigestion. With an "empty" cache, I navigated to YouTube, and QTE worked once more. I don't like to flush my cache on this box because one of the sites I frequently visit uses a "security cookie," and it's a minor pain to regenerate the cookie. Thanks.