I fixed the above by replacing 'xls' by 'xsl' (in rss_feed_v2.0.5_20.10.2007) in function rss_feed_out(), and I found the same typo in 2 readme files. So it was caused by a small typo.

Unfortunately my xsl stylesheet still doesn't work for some reason though. It seems server related because if I copy a sample xsl stylesheet to my local PC it works, but when I copy the same file to my server it does not. Even tried another server and the same xsl file works perfect again.

Does xsl depend on the server configuration?