Adding Facebook Pixel Event results in error

I’m trying to create a Facebook Pixel Event, however I get an error on my site, every time I try to add it. The error states: "Request could not be processed: We could not process your request. Please try again." on the page where I want to add the pixel.

The error appears on my website on the subpage where I want to add the event. It’s a Facebook Event Manager error, as it seems. This is what it looks like on my page (it’s in German though) enter image description here

My website is a WordPress website and I added the Pixel via Google Tag Manager. The pixel itself seems to be working fine.

I googled a lot but didn’t find anything related to this. The Facebook Pixel Helper Chrome plugin shows that the Pixel is correctly installed and working. When I look at the Chrome developer console I see this error however: "Error: Invalid data".

What could be causing this problem? There is literally NO INFORMATION on this kind of error on the web, so I’m really helpless here.

I cleared all caches, tried using other browsers that I never use but that didn’t help either. The weird thing is, it did work only 1 hour ago and now it suddenly stopped working.