free.fr seems to be a pretty restrictive provider (but I guess it's free and you get what you pay for), we get bugreports caused by strange configs and restrictions set up by them quite often.
I had a look at the provided URL and could reproduce the problem but can't figure out what happens. It shouldn't be possible at all to get this message by calling the normal doku.php script, because it is produced by a completely different script.
The problem seems to be caused somehow by the lib/exe/indexer.php script. You should first make sure your data/locks dir exists and is writable by the webserver. If this is the case try to remove the usleep(50) call in the mentioned script. Not sure if it helps but http://faq.free.fr/adsl/4/9/3/8
says the usleep function is disabled at free.fr
My suggestion: get a real provider.