it is very difficult to help you, because I have to analyse the request by the form. First of all I have to say, that I can't reproduce this bug. Do you have enough rights to move pages? Maybe you can get the me source code of your silverstripe installation, and I will try to reproduce and fix this one.
the problem occurs not only for editors, but also for me and I have all rights because I'm an admin (only in this member group).
I have compared this site with another and the bug doesn't exist there... The main difference is, that I have enabled Translatable for multi-lingual. Perhaps this could be a reason?
mhm can't be, because saving and so on is possible and no other problems occur.
I will install it again and check if the problems are already there with my customized system...