Question Upgrade of 1.0.8 to 1.0.9 result in a blank page

Hi i've tried to upgrade the forum to latest 1.0.9.
I followed the procedure (uninstall->install->update links)
But the result is that the forum doesn't open anymore.
Just a blank page (maybe the error is on the server logfile i'll check for that and report as soon as possible).
Revert to 1.0.8 fixed the problem.
Joomla is latest 1.5.10.
Cya

For Kunena 1.0.9 build 1231, dated 2009-04-03, make sure that announcements are enabled to avoid the smiley class bug.

If you mean "Show Announcement" it is "yes"

And the install of Kn1.0.9 was successful? After the install you should see a green box saying something like (I don't remember exactly but..): successful install and a report of your mysql and php version. Do you use any template that may have some conflicting java scripts? Do you use the Kunena lates module, if so try to disable it or install the latest version.

Hopefully your log files can give us some hint.

Yes the installation was sucessfull.
I do not use any add.on modukle for kunena, and the template is the standard joomla (purity).
I've enabled the error display on php now i'll reinstall kunena and hopefully report the exact errors.

Fatal error: Call to undefined function mb_strpos() in core.php on line 30

So i've installed the mb_strings as reported in another post
That fixed the issue.
Thank you all.
K.