Error message when clicking on e-mail link

[pkerr]

About 50% of the time when I click on my e-mail link that notifies me that one of the threads has been updated, I get the following error message.

Not able to open ./cache/data_global.php

This only happens when I click the link that is supposed to take me to the last post in the thread. If I click the link that takes me to the begining of the thread there is never an issue.

Any ideas what's going on?