Hi @paltrainer
Please provide the URL to a page on your website where this is happening so we can better assist you. Also, sharing your Site Health information would be very helpful too.
Basically, you seem to have some iframes on your site like this:
<iframe src="/some-page-to-iframe/"></iframe>
This is not allowed in AMP. URLs need to be absolute and use HTTPS. This would be correct:
<iframe src="https://exmaple.com/some-page-to-iframe/"></iframe>
I recommend you to check out the error details in Search Console, which should show you exactly where such an iframe was encountered. Next, find the offending code on your site and change it accordingly.
You might have some caching plugins or similar active that can cause this, so try temporarily deactivating those to see if it resolves the issue.