I recently upgraded my DokuWiki install to 2007-06-26b. The upgrade worked fine, except for a huge problem: it stopped saving old revisions in attic.
* when displaying the old revisions for a page, the last ones aren't displayed as links and you can't diff them against the latest version (see http://www.rez-metz.supelec.fr/wiki/doku.php/wiki:playground?do=revisions
* notification mails sent by DokuWiki don't contain a diff but the full page (with a + at the beginning of every line... well, it's a diff against an empty file!)
Config: Debian Linux server with Apache/2.2.3 and PHP 5.2.0-8+etch7, latest DokuWiki stable release (2007-06-26b, didn't try on a dev version, don't know where to download it anyway...), $conf['compression'] = 'bz2'.
I tried lots of things and ended reading inc/io.php. On its line 186 I tested changing @bzopen(...) to bzopen(...) and here is the message I got:
Warning: bzopen() [function.bzopen]: 'wb' is not a valid mode for bzopen(). Only 'w' and 'r' are supported. in /var/www/wiki/inc/io.php on line 186
Writing /var/www/wiki/data/attic/wiki/playground.1188997649.txt.bz2 failed
Warning: Cannot modify header information - headers already sent by (output started at /var/www/wiki/inc/io.php:186) in /var/www/wiki/inc/actions.php on line 296
So on line 171 I changed 'ab' and 'wb' to 'a' and 'w' and it started working again.
I didn't do further investigation (now it works for me, that's the most important ;-)) but you should try to see if it is the same with gz compression (rb/wb not working etc.) and maybe consider doing a little bit of OS detection before adding 'b' to the mode: I think only Windows does need it.
Hope this helps to fix this soon... I've seen a few other people on the forum and the mailing list having the same problem, it would be nice to have this bug fixed soon.