The error your seeing is due to a missing attribute that is required in DNN 5.1.x - this is easy to fix!

If your site is on your local machine open Windows Explorere or if on a Server located away from your office, using your favorite FTP tool do the following...

1. Navigate to: "Root Folder Name"/Portals/_default/Containers/"The name of the container your using"
2. or if not loocated at the step #1 location, navigate to: "Root Folder Name"/Portals/0/Containers/"The name of the container your using" 
3. Copy the container that is throwing the error
4. Place the copy in a safe place
5. Back at the folder where you copied the container from, open the container using notepad, notepad ++, textpad, ultraedit (whichever tool you prefer) by right clicking the container and selecting "open with."
6. Once you have the file open and you are looking at the HTML find the "<dnn:ACTIONS runat="server" ID="dnnACTIONS" />" tag and modify it by adding the ProviderName attribute so it looks like this: <dnn:ACTIONS runat="server" ID="dnnACTIONS" ProviderName="DNNMenuNavigationProvider"/>
7. Save the file and move it back into the folder you found it in (if you moved it out of the folder)
8. Reload the page, your error should be gone.

NOTE: This should be done for all the container's loaded into your site 

Cheers!