[TYPO3] Problems with indexed_search on RC2

[Claudio]

Hi all
I'm trying to upgrade a 3.8.1 site to RC2, and have some problems
with the indexed_search extension. 
Every time I try a search, I get a bunch of messages like this:

Warning: Compilation failed: unknown POSIX class name at offset 23 in
/_web/www/vhosts/typo3_src-4.0rc2/typo3/sysext/indexed_search/pi/class.tx_indexedsearch.php
on line 1940

After this, the search is apparently successful: I get all the
desired results.
Any hints?
Thanks in advance

Claudio