The two VPSs are currently on calibre 3.48.0, but the latest is 4.10.1; we should upgrade.
We're also currently using the Debian package, but the download docs say:
Please do not use your distribution provided calibre package, as those are often buggy/outdated. Instead use the Binary install described below.
So we should do that (and document the install and upgrade process).
Bugs such as T244825 appear to be fixed in the new version.