I also had this problem. It seems to be an issue with ScriptNo. I had the extension enabled in Google Chrome, but I had it turned off.
The fix is like you said – to completely disable the ScriptNo extension.
Sorry, it’s because I already fixed this issue a year ago in my fork on GitHub.
https://github.com/cmwelsh/wordpress-simple-page-ordering
Would someone like to merge in my changes? I added some isset() checks to fix this issue for now. I also added a patch to set the menu_order value when a new Page/Post is created.
https://github.com/cmwelsh/wordpress-simple-page-ordering/compare/upstream…master
Let me know if I can help anyone further. I love this plugin. 🙂
Is anyone getting this error in 1.4.2? Can you paste the error?
I can’t seem to make this glitch happen on my install of 1.0.