If a collection address no longer resolves, you are sent to All Competitions with a short message saying the section could not be loaded. There is no error page for this. The site does what it can rather than stopping, which is why a renamed collection reads as something breaking instead of a page that moved.
A collection is a hand-picked list of competitions grouped around a theme, at an address in the form uni.xyz/competitions/section/ followed by the collection's name. They are curated by the UNI team, not generated, so a collection exists only because someone built it.
Why a collection link stops working
Collections are edited over time. The most common reason an old link fails is that the collection was renamed, which changes its address, and the old address is not kept. A collection that was retired behaves the same way. In both cases the competitions themselves are untouched and are still reachable through search and through the tabs.
Collection names are also capped at 40 characters, so a long one is cut short rather than rejected. If you are typing an address from memory or from print, it is worth opening the collection from a link instead.
An empty collection is not the same as a missing one
If the collection exists but currently holds nothing you can see, you stay on the page and it reads No competitions found in this section. You have arrived correctly and there is simply nothing in the list. Being bounced to All Competitions means the address itself did not resolve.
What a collection page shows you
It shows every public competition in that collection, whether it is open for entry or finished years ago, grouped by status. Competitions that are drafts or private are never listed, so a collection can look smaller than the person who built it expects.
Finding the collection again
Search from All Competitions for the theme you were after, or use the filter panel for the discipline or category. Both cover the same ground as most collections.