When the repository archive is enabled, deleting repositories without archiving them first is prohibited.
There are two problems with this: 1. The error message that comes up in this case is just "Repository deletion failed", the only way to learn that you have to archive first is by looking at the server logs. This is compounded by the fact that this behavior appears to be undocumented. 2. There should be a way to explicitly override this - it's cumbersome to always archive a repository, then enable showing the archive, then find the archived repository in the now expanded repo list, and finally delete, especially if the archive is large compared to the list of active (non-archived) repositories. E.g. you have 3 non-archived repos, plus 10000 more in the archive, and you want to delete one of the 3 - you need to pick the one from the 3 (easy), click Archive (easy, but is it really necessary?), then show the archive (easy, but again), pick the same repository now from a list of 10003 (hard!), and finally click Delete (easy). This could perhaps be fixed by adding an "Archive and delete" button to the error message that comes up.