I get the same errors too on all videos and it have happened for over a week now.
It happens on four different systems (gaming pc/htpc/test system/and a xubuntu WM on my unraid server) and it started on all off them at the same time.
I can watch youtube in firefox/opera/chrome on all systems no problem i can also use youtube-dl instead of JDownloader.
I also tried not even opening a youtube page at all (FF/Chrome/opera) or trying too download any youtube video (JD/yotube-dl) for 50-55 hours but no change at all.
For any that have this problem try using youtube-dl (google it) as a work around until JD is fixed and the simplest way and the way i use it is this.
Put the youtube-dl.exe in the folder you want to download too then make a txt file named youtube.txt that you copy the youtube links into (playlist should work too but have not tried it) save it.
Open CMD in windows go too the folder you want too download too and use this line
Code:
youtube-dl -a youtube.txt
Filename is only title tough.
If you want channel name/date/title in the filename use this line
Code:
youtube-dl -a youtube.txt -o %(uploader)s-%(upload_date)s-%(title)s.%(ext)s
And if someone know how to get spaces between the different sections (uploader/date/title) instead of - let me know, have not got that too work at all even _ would be better but that does not work either.