I was getting the above error message for the document manager only, all other dialogs were working properly. I have spent the entire day painfully re-creating the entire project from scratch testing along the way to see what broke the document manger. It turned out to be one of the most bizarre things I have seen.
The error occurred if I had a file called “ClickToPlay.png” in the “images” sub folder of the site. I tried it with multiple different files to ensure it wasn’t the file itself and it turned out to be the file name causing it, it didn’t matter what the file was. I have now renamed the image and the project is working.
To make things stranger, I added a “ClickToPlay.png” image to another project I have running the same version of rad controls and it didn’t break the document manager. I can’t make any sense of this.