WordPress 2.1 has “entered beta”:http://wordpress.org/development/2007/01/wordpress-206/. If you’re planning on using K2 with WP 2.1 beta, you’ll need to install the “svn version”:http://code.google.com/p/kaytwo/source. K2 0.9.1 is *not* compatible with WP 2.1 beta.
If you’re having problems with Sidebar Modules or Rolling Archives in K2 0.9.1, you can try upgrading to the svn version. We have made substantial improvements and bug fixes with the svn version. The problems you’ve encountered might have been fixed. If not, please post a “bug report”:http://code.google.com/p/kaytwo/issues/list.