Navigation menu items blocked – cannot move items

I am at a complete loss here!

I have a main navigation menu, with submenu items etc., the usual thing.

Now, for some very strange reasons, I cannot move those items around any more. No drag and drop, zip! I can add new items, but they remain at the bottom of the list, not movable either!

I have no freaking idea as to why this happens! I disabled ALL plugins, changed themes etc. nothing helped.

As it is time now to change that “dummy menu” I created at the beginning of my project it would of course be useful if I could actually link to those pages I created!

As a last resort, would it be a possibility to modify the entries in MySQL directly?
Any help is really appreciated. Thank you