Based on comments in issue 0002953 and on the php doc site, try adding a check for zlib before attempting compression. This doesn't seem to be needed in all versions but maybe it will help the people who are having problems.

• core/compress_api.php
  (compress_is_enabled): add a check to see if zlib is available

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2102 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9