After a many strange, wrong symbols in the text on my sites I was finally able to fix the problem myself, if anybody has the same problem, with version 3.15.3 you just apply this change:
file advertising-manager/advertising-manager.php
line 123
replace:
return preg_replace_callback($patterns, ‘advman_filter_content_callback’, html_entity_decode($content));
with
return preg_replace_callback($patterns, ‘advman_filter_content_callback’, $content);
in brief: that html_entity_decode() just breaks the code.
My recommendation is to NOT perform a mysql search+replace.
With this simple fix all ads display beautifully, e.g.:
https://www.simplestgallery.com/what-is-simplest-gallery-plugin/
Long live Advertising manager, by far the simplest and most intuitive plugin for ads around!