If the site runs ok with ABP removed that indicates ABP is the problem.

I use UblockOrigin, if I disable that for the site in question { vcp-forum.de ) then the warnings go away, I need not uninstall UBlockOrigin.

Hi, there is no 'hidden' ad-blocker in Firefox, but it's possible something might have been remembered from when ABP was enabled, so you could try deleting ALL references to the website in question, as shown under the heading 'How do I remove a single website from my history?' in this article - Delete browsing, search and download history on Firefox, then try the site (not from an old bookmark) again. Note: It may take a few minutes to complete the deletion.

Even uninstalling ABP did not stop the message. Anyway I have uninstalled ABP now and installed Ublockorigin and disabled on VCP. No change. I have allowed ALL history in the options now thinking that might be the problem. No change. I am running google analytics and I had this disabled, again no change. VCP displays the message. I have attached a screenshot of all running addons.

I'm seeing that message as well. You see this when a Google ads div has height=0. I'm not seeing any messages about blocked content, so it could be that there is currently no such adds available and you can ignore it.

<div class="panel headerads">
	<div id="gandalfads">
	<script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>
 .....

<script>
$(document).ready(function() {
if($("#gandalfads").outerHeight() === 0)
$("#displaywise").html("<div class='panel noscriptcss noscriptcssresponsive'><p class='warntext'>Please disable your Adblocker!<br>Remember: <a href='https://vcp-forum.de/donation'>Donators</a> do not see the ads!</p></div>").show();
}); 
</script>

If you are visiting the site in a private window, you'll get that messages until you disable tracking protection. More info: What happened to Tracking Protection?.

Solved it. The extension blocking the advertising banner was GOOGLE ANALYTICS. When disabled the page displays as normal.