This is happening here, at SMF, but I also just tested at my fresh RC4 test site (Github).
Once an image is inserted into a post, you cannot go back and modify the post. The option to do so is there... it's just that nothing is saved. You cannot preview your modification, you cannot save your edit. The first attached screenshot (and I won't "insert" it) is my attempt to modify. Oddly, for whatever reason, the field turned yellow after attempting to modify, and clicking on it would not make the yellow go away. (That did not happen here on this site.)
The second screenshot is the error this attempt to modify threw. I wouldn't suggest that it is a bug, were it not for the fact that it is also happening here.
EDIT: I seem to be able to modify as long as I have not clicked "Insert".
I can replicate this. Reported here with the cause: https://github.com/SimpleMachines/SMF2.1/issues/6989 .