At first I thought that it was because of this issue because I accidentally did something in the body without noticing, so I clicked Cancel and then tried to edit again and it did the same thing. I tried again and again and always got the same problem. So I'm sure that I didn't touch the body, only the tags.
The strange thing is that I successfully submitted another retag today and several retags in the past without getting this problem, and I regularly see tag edits on Stack Overflow where I have the privilege to review suggested edits, so normally edits just editing the tags should be allowed.
Is there a reason why I couldn't submit this specific retag without editing the body while I've submitted many other retags in the past without any problem? If this is a bug, please fix it.