Yes - the add-on is totally removed.
And I found the real culprit!
It were certain rewrite rules in .htaccess:
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -l [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^.*$ - [NC,L]
RewriteRule ^(data/|js/|styles/|install/|favicon\.ico|crossdomain\.xml|robots\.txt) - [NC,L]
RewriteRule ^.*$ index.php [NC,L]
Redirect 301 /showthread.php
https://www.nationaalcomputerforum.nl
On the server I looked specifically for this rule: (data/|js/|styles/|install/|favicon\.ico|crossdomain\.xml|robots\.txt) - [NC,L]
The section after data/ is non existent!
And after removing this whole section from .htacces the contactlink is now completely restored and functions!