The 7.97 distsrv probe is not compatible with hub versions earlier than 7.96.
Two options available to resolve this issue:
1. Redeploy the 5.40 distsrv probe that is available as part of the 8.51 UIM install
2. Upgrade all robots to the 7.97hf3 version (or later if a later hot fix is available) and hub probes to the 7.97 GA release.
The 7.97hf3 robot_update hot fix can be downloaded from the CA UIM Hotfix Index