YouTube embedded video shows blank in Safari firefox says file not found

Dear all,

I have embedded YouTube and Vimeo videos in a website in adobe dreamweaver 5.5, MacOS 10.7.5. They show blank in Safari, and firefox says (translated from German): "File not found. The files under (embed code) could not be found." My slightly older embeddings (also "i frame") in the same website work fine though.

I have updated today Safari, Flash Player, and Java; reset both browsers, nothing changed.

Have spent ours googling this already, no help.

Thanks for helping.

Tobias