Do and check the following to fix the problem:
1. Did you earlier create and then delete some nodes under the folders you are working on?
2. Did you make too many changes without saving after each change?
3. Close the browser. open a new window. Clear the cache to be sure.
Redo the changes (you could choose different node names) and save each time after every change. Doing this seems to solve the issue in most of the cases
You should be able to save now.