Navigation broken in Modern Team Site

Hi,

the navigation of our team site seems to be broken since this morning:

When navigating to the sites root (by navigating to the url), the site loads and looks fine. The first navigation through the left-hand navigation works as well. However, every subsequent navigation attempt exhibits the following behavior:

• When first clicking an item in the site navigation:
  • The page header updates (title, background image)
  • The page is highlighted correctly in the navigation
  • The *content remains unchanged* (from the "previous" page)
• When clicking a second time on the navigation item (same link):
  • The page reloads fully showing the proper header and content
    (however, no ajax/"SPA-like" content update)
  • Highlighting the current page is broken if the page isn't a top-level page
    (which is a follow-up error I guess)

Note: This issue also occurs when the second navigation occurs through a link displayed by the quick links web part. Here, in addition, the second click on the link causes the linked page to open in a new tab (making the experience even worse).

We're not aware of making any changes to the sites configuration. Apart from adding an spfx extension (Application Customizer) we only edited content. Just to be on the safe side, I've removed the extension again, but the error problem is still present.

The ServerVersion as obtained via powershell by getpnpcontext is 16.0.8008.1220

Any ideas what could be the cause of this issue or how to resolve it?