Haven't been in contact for a while now.
It's London, now. Right?
BTW, the version shown in the page footer, SMF 2.0.15, was released Nov 19, 2017.
Current version is 2.0.17 dated Dec 30, 2019. Sometimes when things stop working, it's best to start by getting the most recent release and testing it. YouTube makes lots of changes, so it's reasonable to assume the software that creates the YT embedded connections might not be using the most current requirements. There are several YT embedding mods (modifications) that are available, and I have no idea which, if any, are installed here.