I have done a hard reset on the phone (home + power until the phone stops), upgraded firmware, done a restore of the phone, and tested YouTube using Safari.
Safari allows me to login with no problems (on both the iPhone and my Mac)... but the YouTube application from Apple doesn't.
So no one knows how to f#*{in' fix this?!?! This iPhone 4 is bullish*t to me now. Too many problems and I don't feel like exchanging this phone for another.
Same issue here since yesterday with both my iPhone 4 and iPad 3G , everything works fine using Safari though , is it a Google-Apple thing to push Android over iOS ? because YouTube is part of Google ....
I am having same problem. It started when I used Google for YouTube. And I only did that because it was a way to share liked videos on my Facebook page. I prefer to go back to Apple's YouTube app. So, I went into my account settings for Google YouTube and there is an option to link or unlink Google and your YouTube account. Mine was linked. I unlinked. I went through all the iPhone restarts and am still having the problem.
I was having a different problem with youtube where it was asking me to log in and then when I entered the user name and password it would redirect me to the youtube web site to log in.
I solved the problem by displaying the running app bar (double click the home button) and then deleting the youtube app.
Not sure if this will help but you may want to try deleting from the running app bar all apps that have anything to do with youtube.
FWIW I was having the same problem yesterday on my 3Gs (iOS 4.0) but seems fine now. I couldn't get in via app or Safari so it might be on YouTube server end.
I called AppleCare on this one. Issue resolved. No concrete explanation as to cause. My problem existed today when I was on 3G with no access to Wi-Fi. There was a spot during my travels where 3G coverage died out. At home, where I had Wi-Fi, AppleCare tech asked me to switch over to Wi-Fi. When I logged in, I got in, but it was a little sluggish. I did a power down (pushing top button down until you get the red slider, then sliding it over to shut down). Problem is gone.
i have an iPhone 4 with ios 4.0 i was having the same problem since 2 days ago and i couldn't figure what it was until today what i did was hook up my phone to a computer at work and iTunes loaded up then i unlocked the phone and gave it a moment the while my phone was hooked up to the computer and iTunes was loaded i loaded the YouTube app and signed in and it worked with out a problem. i even signed in an out a couple of times and still fine.
sorry about the run-on sentence i hate grammar. 🙂
I see this post is still unanswered. Synster Gates, did you ever get resolved? It seems that people end up resolving this on own using different methods but with no clear explanation as to the cause.