When using a certain number of plugins and a rather large Wiki page the lexer may hit the PCRE backtrack limit ( http://uk3.php.net/manual/de/ref.pcre.php#ini.pcre.backtrack-limit
). In my case the compound Regex was 2257 byte consisting of 54 individual Regexes and the Wiki page 100039 byte.
The failing preg_match() is in inc/parser/lexer.php, line 115, function "split". Calling preg_last_error() right after it has failed returns a value of 2, PREG_BACKTRACK_LIMIT_ERROR. This could be used to issue a warning to the user. This failure simply results in DokuWiki outputting the unparsed Wiki page and is thus pretty hard to figure out. Getting to the root of the problem took me a few hours.
Increasing pcre.backtrack_limit in the php.ini from the default 100000 to 1000000 fixed the problem, no further memory or other problems occured.