To make SMPlayer play the "forbidden" YouTube videos via SMTube I had to change the following SMTube setting:
View -> Settings -> Players -> SMPlayer -> Edit -> uncheck "This player supports video sites" + click OK to confirm.
Worked out of the box, no update via mx15 repo needed.
@Aleph: Thoroughly enjoyed your interpretation of Piazzolla's Adiós Nonino. Heard Piazzolla's quinteto play it live in a concert in Frankfurt more than 30 years ago ...
SMPlayer was working fine after the last update, anyway I have unchecked "This player supports video sites"
I am glad that you enjoyed my interpretation of "Adiós Nonino" Thank you!!
You was very fortunate been at the Piazzolla's concert, I envy you...