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That seems odd.

What version of WordPress do you have and what type of user account do you have?

I’m using verison 3.9.

With the administrator role (the first user you make when you install a fresh wp-site)

I’ll set up a new site for myself and see if I can replicate the problem – and report back.

When I make a new site, it works.
No error or something else.

That’s good news.

What plug-ins are you using on the site where Rotating Tweets wasn’t working?

You might also want to install something like Wordfence Security on your previous WordPress installation and see if it identifies any problems.

I was using my own plugins which worked all fine. Then I got the error and I tried to deactivate all the plugins and activate Rotating Tweets.

But that gave the error as well. Next I tried to use another plugin which gave the same result. I’ve tried that one without any other plugins running but same thing.

On my new site however, everything works without a problem. Even the previous error I noted (about the admin_menu hook) didn’t show.

I couldn’t figure out where the error came from because I haven’t opened a file outside the wp-content folder.