I found the reason. In /index/templates/jd_boston/html/com_virtuemart/sublayouts/addtocartbar.php
there are no input buttons for plus and minus, only image tags (see addtocartbar tmpl JDBoston.jpg).
When I replaced the img tags with the input tags as the where in the original addtocartbar.php, the buttons worked, but I lost the images for + and -.
So I inserted two button tags with img included and now everything works fine, but the images do not appear centered and they lost their red color. (see plus minus new.jpg)
JoomDev is not affiliated with or endorsed by Open Source Matters or the Joomla! Project. The Joomla! logo is used under a limited license granted by Open Source Matters the trademark holder in the United States and other countries.