Hello Michael,
I've this problem too. I noticed that it happens for some pages which are created by a script. I've posted the page where I get the error here:
http://pastebin.com/40wVX0PT
Result of the indexer.php call (lib/exe/indexer.php?id=info:switch_doku_access&debug=1):
runIndexer(): started
Indexer: getting the PID failed for info:switch_doku_access
runSitemapper(): started
runSitemapper(): finished
sendDigest(): started
sendDigest(): disabled
runTrimRecentChanges(): started
runTrimRecentChanges(): finished
runTrimRecentChanges(1): started
runTrimRecentChanges(1): finished
###############################################################
Edit: It was working with Release 2010-11-07a "Anteater"
I hope that this helps to solve the issue.
Please let me know if you will need further information.