Extreme slow "Writing substring data ($P) to database."

Our system makes intensive use of Oracle Text. Normally everything is working as expected, but since yesterday there is one server which "hangs" on Writing substring data ($P) to database during the indexing process. I enabled logging to get some more information, but specifically at this point there is no more information logged other then this single line. All other activity on this machine is "business as usual" and there is no slowness measurable for any user.... Only the indexing of documents does not proceed. This will become a problem in the coming days....

the sqlstatement for the session is: