This is not a bug but working as intended. Gaps in numbering are not supported.
This is not working because the numbering is not successive. This whole
configuration implementation is of course nothing to be proud of.
plugin {
#setting_name = value
# archive folder
mailbox_alias_old = Archive
mailbox_alias_new = Archief
mailbox_alias_old2 = Archive
mailbox_alias_new2 = Archief mappen
mailbox_alias_old3 = Archive
mailbox_alias_new3 = Archived messages
mailbox_alias_old4 = Archive
mailbox_alias_new4 = Archived mail
mailbox_alias_old5 = Archive
mailbox_alias_new5 = Archives
# spam folder
mailbox_alias_old10 = Spam
mailbox_alias_new10 = Junk
mailbox_alias_old11 = Spam
mailbox_alias_new11 = Junk E-mail
mailbox_alias_old12 = Spam
mailbox_alias_new12 = Ongewenste e-mail
# drafst folder
mailbox_alias_old20 = Drafts
mailbox_alias_new20 = Concepten
}