It seems that the zimlets were not updated correctly on our system.
Therefore the new posix zimlet failed due to dependencies/errors on older core zimlets. redeploying them fixed the problem and the panel showed up again.
Make sure you have a working backup BEFORE! Especially of the /opt/zimbra/zimlets-deployed folder.
what we did (no warranty disclaimer comes here):
zmzimletctl deploy /opt/zimbra/zimlets/com_zimbra_cert_manager.zip
zmzimletctl deploy /opt/zimbra/zimlets/com_zimbra_adminversioncheck.zip
zmzimletctl deploy /opt/zimbra/zimlets/com_zimbra_bulkprovision.zip
which updated to the current ZCS 7.x core zimlets. since then the posix tab shows up again!
