The normal way in which the optimiser is installed involves adding a couple of lines in the php.ini file that point to the offending .so file. This looks exactly like what would happen if the optimzer .so file was removed but the setting pointing to it was still in the php.ini file.
Ask your ISP to check.
The only other slight possibility, I guess, is that there is a broken .htaccess file involved somewhere, which is also worth a check.
Hardware and performance
Zend?
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