Comment 8 for bug 1767453

I think I have found the culprit. It is a package called "autofirma", distributed by the Spanish Administration to perform digital signatures with the official certificates provided by the government.
Apparently, there is some kind of incompatibility between the files generated by this package and the postinstallation script of this version of ca-certificates.
I have just removed (purge) the autofirma package, then update ca-certificates to 20180409 without any problem and, after that, reinstalled autofirma (probably I may need to uninstall it again when another ca-certificates version comes out).
As autofirma is not an ubuntu-provided package, this should not be considered a bug after all. In any case, the information may be useful for any other Spanish user finding the same problem.