In the zpBase theme folder find the file [i]functions.php[/i]
On lines 240 and 242 add trailing slashes.
Line 240 becomes

see added trailing slash after [i]slideshow[/i])
`$slideshowlink = rewrite_path(pathurlencode($_zp_current_album->getFolder()) . '/' . _PAGE_ . '/slideshow/', "index.php?p=slideshow&album=" . urlencode($_zp_current_album->getFolder()));`
Line 242 becomes

see added trailing slash after [i]slideshow[/i])
`$slideshowlink = rewrite_path('/' . _PAGE_ . '/slideshow/', "index.php?p=slideshow");`
Don't ask me why but this seems to work ;-)