This is most probably a problem in the less compiler that is included in DokuWiki in combination with your PHP version. Could you try changing the
!=[/m] in [m]inc/lessc.inc.php[/m] into [m]!==?
This blog post explains the problem and was also the inspiration for this fix. I've just tested this both with the DokuWiki CSS and the test cases from LESS itself and I didn't notice any problems.