Hi @FreekR
I can reproduce this issue on your website as well, please update the Revolution Slider plugin to the latest stable version, and try these suggestions mentioned here:
https://www.themepunch.com/faq/troubleshooting-tips-for-5-0/
What I believe it could be related to a bug in Revolution Slider described here, however, I’m not exactly sure if it’s fixed or not, what makes me feel like it’s this one is that if you tried another slider with images only, it works fine, it’s just this one with YouTube code that throws this error in the browser console.
Let me know how it goes,
Thanks.