The configuration option

modules/com.openexchange.custom.spamexperts/module

will be renamed to

modules/com.openexchange.spamhandler.spamexperts/module

in

/opt/open-xchange/etc/groupware/settings/spamexperts_gui.properties

because the name of the plugin must match the name of this option. If that is not the case, the plugin does not work.