I've found a workaround by disabling the Last Modified tag in the SEF patch extended, and this solved the problem, but this is a valuable thing SEO wise, so I hope someone will find a better solution for this problem. And I just can confirm here, thet upgrading from 1.5.0 to 1.5.1 revealed the problem. I have it "successfully" reproduced in 3 different sites, for example on the one wich hurts me most,
www.joomla-tips.org/