Youtube videos don't embed properly anymore because some code was removed by them. It is necessarily to manually type in
http:
after src' and before the forward slashes in the SECOND place where the URL appears
Go down to the MIDDLE of the embed code and make sure you type in http:
between src' and //
There are two spots where the URL appears, it is necessary to type in http in the second instance (where there is an src before the URL)
See PUblius's comment which explains it fully
See Publius2008's comment for a complete explanation and visual.