Saving session to database

I'm having a problem with webtrees ... http://webtrees.net/
My copy is running on http://webtrees.lsces.org.uk and you will see that it is 
throwing an error relating to the session handling.

Running Apache 2.2.22, PHP5.4.14, MySQL 5.5.31 ( http://lsces.org.uk/phpinfo.php )

If I comment out the session_set_save_handler() section of session.php then I 
can log in and use the site. Have tried reworking that as a 
SessionHandlerInterface without any success. As far as I can see, the 'write' 
function of the handler is not being called and while there are various notes 
about problems with PHP5.4 blocking access to the write and close functions, 
none of the fixes I have tried have resulted in a working setup.

Anybody seen this problem with PHP5.4?

http://lsces.org.uk/hg/webtrees/file/dcf6ff358108/includes/session.php#l334 is 
the code that seems to be failing.

-- 
Lester Caine - G8HFL
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