I also got a problem on my website with yvcomment. The website was an RC3 install and I tried to upgrade the yvcomment when I saw the message that I have to upgrade J! to the nightly build. I copied the files and installed the latest yvcomment and end up with
Fatal error: Cannot redeclare class jfilteroutput in /var/www/vhosts/xxxxxxxxxx.nl/httpdocs/libraries/joomla/filter/output.php on line 25
Of course I can install J! again as stated before but isn't it better to find out why this error occurs?
1. First of all, it is generally not correct to copy New Joomla package over older one, because Joomla is constantly refactored now and sometimes classes are being moved from one file to another. So, if you don't need to move huge amount of Joomla! data from old site to new one, my advice is to reinstall Joomla! and don't waste time, trying to figure out, what causes what errors.
2. Just because Joomla! is constantly changing, I have to change yvComment accordingly. So, it is natural, that one version of yvComment doesn't work with older (or newer) builds of Joomla!. In order to help you to figure out this version incompatibility, I implemented special Joomla! build number check.
3. As you may expect, there is no point (and maybe it's impossible) to make yvComment compatible with different builds of Joomla!, because Joomla! itself is so incompatible between builds (on the level of Joomla! API, that is being used by yvComment)