No changes permitted - Something wrong?

Hi everybody,
My Omeka version is 2.0 with Seasons theme.
(I previous installed last 2 rc versions.)

I notice today that it is impossible to modify or to create the links to navigation (the labels...) on the "appearance/navigation" menu. 2 warnings are displayed:
"The navigation settings were not saved because of missing or invalid values. All changed values have been restored." "Navigation links that have undeleteable sublinks cannot be deleted."
- I notice a second problem: modify the size of images on the menu "appearance/settings" doesn't have any consequences.
Many thanks for suggestions for solve the problem.
Best regards

Can you not make any changes to the navigation, like reordering? Or are there some changes that work and some that don't.

On the image sizes, I'm pretty sure that the images sizes define the sizes of the derivative images when an image is uploaded, so changes will only affect images added after those changes are made.

Many thanks for your help.
- NO, any changes like reordering permitted.

- I'm not sure for the image size. It's seems to be blurry if I erase an image, change the definition and upload a newer. But it's not very important for me at this moment. I tried many sizes and definitions (in pixels and dpi) for my maps corpus. I just regret the "square" format for the thumbnails because all my maps are rectangle.

Not that some of the themes specify maximum images dimensions in CSS. This can make it hard to tell if the underlying image has the new dimensions (unless you're using a "high DPI" device that normally has to scale up images).

I'm a little surprised at the error message you're getting from Navigation. It should only do that if there's a plugin-added navigation item being removed somehow. I can't personally recreate your problem. Are there any more details you can give? Did the navigation panel ever work for you?

It works when I installed for the first time Omeka but it is failed since the 2.0 installation.
Can I verify the integrity of the files (right number) or try a newer installation?
Could it be a problem of rights?
I manually change some parameters in the css file (Seasons).