After creating the PDF, I get this error message:

warning: missing value for property 'content'

Any tip on how I could solve the issue?

Thanks

Do you have a style sheet using the "content" property?

yes, for example:

@page dedication:blank {
  @bottom {
	content:"";
  }
}

I used this property quite a lot in older Prince versions.

Perhaps there is a <style> element or style attribute in the document that uses the "content" property without a value, like "content:;"

Prince did not used to warn about these kinds of syntax errors in CSS but now it does.