Vimeo Link doesn’t find it’s target (iframe)

The CMS Questions
1

My code is perfectly working local. Only the target of the vimeo links suddenly don’t find their iframe as soon as the page is uploaded. Any hints?

2

Any errors in console and/or network tab?

3

console says:

null:1 GET https://myurl/null 404
Image (async)
changeVideo2 @ de:922
onmouseover @ VM750 de:1