Deleting a single cache doesn't seem to work anymore

I’ve now seen this multiple times: A build breaks because something in the cached contents is wrong (an error from the developer), and so we want to remove the cache and try a “clean” build.

This used to work easily through the Travis UI – find the repository, settings, caches, pick the offending cache and press the trashcan icon.

Since “a while” it seems that that doesn’t do anything anymore, instead:

  1. Reloading the caches page shows the cache again
  2. A build will continue to pick up the cached contents.

Only when one deletes all caches do they actually vanish.

https://travis-ci.com/github/Collaborne/material-kanban/builds/189304149: A build that used a cache.
https://travis-ci.com/github/Collaborne/material-kanban/builds/190443889#L184: A build I manually triggered after removing the cache through the “trigger build” feature. Clearly it picks up a (the) cache, as per the UI that after reloading also shows the cache again.
https://travis-ci.com/github/Collaborne/material-kanban/builds/190444259: A build after deleting all caches.

Impossible to delete a cache for branch 'c++17' surmises problems with branches with ‘/’ in them. In my case that is not the case, the problem is for the master branch. BUT, my cache does mention the NodeJS version – ‘lts/erbium’, without escaping.

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