Can navigate from A to B and from B to C, but not from A to C [updated]

So I have this rather big menu system (~ 40 links on the left-hand side of the screen, Ajax navigation), and one of the linked pages causes problems. After visiting it, some - but not all - of the other links only cause a reload of the current page when clicked. But to simplify, let's just assume there are three pages (in addition to the menu): Layout, Countries and Vat.
When visiting Layout, one can click the Vat link and it works flawlessly. The same goes for going from Vat to Layout. However, when at Layout and pressing Countries, that just causes Layout to reload.
When I try to track the execution of what happens I see the following:
I click "Countries" in the menu. This fires Page_Init() in the Menu.cs. Then, execution moves to BasePage.cs (which Menu.cs inherits) and LoadPageStateFromPersistenceMedium(). However, after this last method has been executed, execution halts. Normally it continues to BasePage.OnLoad() and then Menu.Page_Load( ...