[Theme: rocktree] previous_posts_link / next_posts_link seems to be broken…

i just installed the new wordpress release 2.6.3-r1 in german and noticed that the previous_posts_link function seems not to build the permalinks correctly.
my permalink structure is as follows:
/index.php/%year%/%monthnum%/%postname%/

normal should be this right?
https://6×60.breelr.de/index.php/page/2/
but what’s getting linked is this: https://6×60.breelr.de/index.php/Index.php/page/2/

what’s the deal? bug in release 2.6.3-r1 or theme related error?

thanks for any help ??