Chrome ships with it's own Flash Player version.
Make sure your chrome installation is up to date by doing the following:
Click Chrome > Preferences > Help
Does chrome indicate that it's up to date?
I found a fix here: http://forums.macrumors.com/showthread.php?t=1445213
"To fix this, type in "chrome:plugins" into the address bar to bring up Chrome prefs.
The list of plugins should say "Flash - Version: 10.3.183.23 Download Critical Security Update".
With this version already installed, click "disable" on the toggle for Flash. It will go grey (inactive).
Try loading youtube..."
Yeah, it's up to date but says "This computer will no longer receive updates of Google Chrome because it's running Mac OS X 10.5"
I've just tried this, but a Youtube video now says "The Adobe Flash Player is required for video playback" and there's a link underneath.
But this link takes me to the update download page - which I've already tried.
Which version of Chrome are you running? 21.0.1180.90?
Now that I've tested more youtube videos, I'm seeing the same error. It seems random, in my case, some videos work, some don't. I suppose one other option is to allow out-of-date plugins in Chrome:
But, other ideas are appreciated.
Okay great, that seems to be working for the mo. Thanks a lot!
Apple is no longer maintaining OS X 10.5.x with security updates.
If you have an Intel machine, we would recommend that you upgrade to at least OS X 10.6, which continues to be supported by Apple and most browser manufacturers.
Because security updates for the OS are no longer available, most browser vendors have also followed suit:
You can continue to run OS 10.5, but your system will be vulnerable to multiple, publicly-disclosed security issues at the OS, Browser and Flash levels. We highly recommend that you update to the latest available OS version for your hardware.