Check with your hosts to see if any of those other sites are on the same server. In which case the errors appearing in your logs may be the result of a busted server.

It appears from the log, and your comments about what they did with regard to Zend and Zend Optimiser, that they have broken their PHP installation, probably as the result of a badly applied upgrade. It's possible that they have a version of Zend and Zend Optimiser which is not matched to the version of PHP they are using (they have to match).

Vger