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Try switching to a different theme. Many hosts block the timthumb script following serious security issues last year.

Thank you esmi for your quick reply.

Unfortunately I can’t switch themes right now. I’m working on a big redesign for the blog but it’s not ready yet.

Any other way I could fix this?

Thanks again.

There is no magic bullet in these situations. This is just the start of basic troubleshooting and is a process of elimination deliberately designed to locate the root cause as quickly as possible by first removing the most obvious and common culprits via a series of (often) temporary steps.