I tried to reproduce the problem on my own site and I don't have any problems inserting the same URL as in your screenshots, I'm not sure what is causing that validation error.
You can try inserting the shortcode manually rather than using the GUI, in this format:
As Nigel was able to insert the URL using the user interface fine it wouldn't be considered as a bug.
Also as he mentioned the User interface will generate the shortcode for the logout redirect, however you do have the option to add it manually like this below.