This sounds like it may be due to a bug that was fixed in 5.2.
There was an issue moving items around in the hierarchy and the database not being updated properly reflecting in the menu not loading things properly.
Yours sounds a little more severe, so I'm not sure. You might try to do a test install locally of 5.1.1 and get steps to reproduce the issue. Also try it with a local copy of 5.3.1 to see if it has been fixed in the latest version.

While testing I found it best to avoid using "Install" as a Page Name.
I was seeing if it was broken in 5.3.1 [which I couldn't get it to reproduce, so it may be fixed in the latest version] and was putting in your Page Names and if you have humanfriendly URLs on in web.config, "Install" as a Page Name causes a "Resource cannot be found". The bug is at http://support.dotnetnuke.com/issue/ViewIssue.aspx?id=12173&PROJID=2

Has anyone created a Gemini issue for this?

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Chris Hammond
Former DNN Corp Employee, MVP, Core Team Member, Trustee
Christoc.com Software Solutions DotNetNuke Module Development, Upgrades and consulting.
dnnCHAT.com a chat room for DotNetNuke discussions

In the name of saving everyone the two days of hell and chasing my tail I just went through, this bug reaks havok with JDMenu from AllDnnSkins. Took forever to track down because I just bought the skin and assumed it would work, since all the rest of my skins do. I didn't even know I was in the messed up state that this bug causes because everything works fine and has for months. If you get an exception in JDMenus PageInit, look here first.