If you see the error coming up from one specific folder then the corrupt entry could be located there. If you only see it when you click on the All Computers
group, then it is a machine that is not within a group. If you add machines into a test group folder a few at a time and then click on that group, eventually you will add the machine that has the corrupted value. Narrow down the results until you find the machine that is the cause and delete the machine. The machine should then re-add itself without the corrupt entry.
Deployment Server 6.8 & 6.9 all versions.