Comment 1 for bug 1969757

There's not much obvious from logs, but my best guess is that this may be because of bad template substitution in a charm (that I fixed a little while ago in https://code.launchpad.net/~cjwatson/rutabaga/+git/rutabaga/+merge/418939, but hadn't got round to rolling out to production yet) that means that the builder proxy authenticator's database of valid tokens has been bloating without bound ever since it was installed, which is somewhat likely to cause random failures like this.

I've filed https://portal.admin.canonical.com/C149735 to get the fix rolled out. That should also give us better access to Squid logs, giving us a better chance of debugging future problems of this kind.