I actually found the problem last night. After I made this thread, I noticed I was having the same problem on other sites too. Having Googled it, the general consensus was that I should do exactly as you advised. However, I didn't have this add on installed. However, I noticed another add on (YouTube Downloader or something like that) that I don't remember installing. So I removed that and that did the trick.
Cheers, Gerry 🙂