What appears to be random, signs on the DHD will disappear.
A stargate with a missing DHD sign will allow incoming activation and use, but without the DHD sign no outgoing location can be selected.
The log displays this message for stargates with missing signs on server startup, "[WARNING] Unable to get sign for stargate: Castle and will be unable to change dial target." with Castle being the name of the stargate on the Public network.
The missing sign issue affects all networks, not just the Public network.
I am able to place a new sign one the DHD and all works fine, but for the non admins this is a problem as they can not alter stargates. The sign on the stargate itself is always still in place, which makes it easy to place a new sign on the DHD as the sign name and network are shown on the stargate sign.