Shouldn't be, so who changed what to make this happen?
Decision time, you could spend a little while searching for an answer, which I suspect is likely to be external to Moodle, and try to fix it. Or you could just replace the code-base.
If you replace the codebase and it happens again, there is an issue with your setup, your server, your host or whatever. Talk to your server admins before doing anything though. Think it through, plan your response and consider the time it will take to do either.
Nicer to be able to find the real issue, but likelier to be an environmental thing.