Hi Alexander,

I also faced this issue early on. I was trying to use Kaltura videos in my to-dos instead of Youtube, since Kaltura is what my org uses for internal videos. Within Kaltura, there is a Share option and within that option there is three options - Link to Media Page, Embed, and Email. For my to-do's to work I need to copy the Embed html code from Kaltura (which includes the iframe code within it). 

Then I paste that code as the HTML source code. You paste as HTML source code instead of using the URL type on Insert/Modify Video option in the TinyMCE / WYSISWYG.

I attached a copy of my source code, in case there is something telling for you there.

Good luck!