Heiya Sigbert, I would like to suggest that this extension may be invoked by using R.php instead of Rext.php. To have a file name deviating from the directory name is non standard within the MediaWiki world and is always likely to cause confustion. Cheers
Topic on Extension talk:R
Hmm, by looking at the instructions I suspect that the R software already uses R.php. This would change things. I tried to have a look but somehow I was not able to extract the tar provided. Perhaps an info that the R software is a precondition and should be downloaded separately would be good.