The Horde Groupware Webmail refuses valid S/MIME certificates…
In the first place, user uploads a valid certificate into the webmail system in order to be able to sign and encrypt emails. Horde replies with the affirming message, that the certificate was successfully uploaded. But it was not and after re-logging into the webmail system, the following error occurs:
A fatal error has occurred Horde_Prefs_Scope::serialize() must return a string or NULL Details have been logged for the administrator.
or in the Czech language:
Vyskytla se závažná chyba Horde_Prefs_Scope::serialize() must return a string or NULL Podrobnosti byly zaznamenány do log souboru pro administrátora.
This error is caused by non ASCII (UTF-8) characters in the certificate and does not have a solution. In other words: Do not upload your certificate. Or better: Do not use Horde at all!