Ok, I used a hack
colmask ul ul {display:block !important;}
But inspecting the element in the browser, reveals still a mysterious element.style that is display:none :shock:
It is not visible in the source, somehow gets that property via javascript or else and is beyond my comprehension. I installed fresh joomla 1.5.20 and installed the fresh template. Then I replaced the whole template folder with the one I had previously modified (the looks you see now) and which is working fine on another identical server. The only difference is that the problematic site runs joomla 1.5.20 and the working site joomla 1.5.17