[Cause]
There is a problem with the setting of which slave device takes priority among the master and slave devices.
[Countermeasure]
On the master connection-type setup screen, check that only 1 slave has "Priority" set to "Yes". On the master link-type setup screen, check whether "Priority" is set to "Yes" in the Robot Link advanced settings of the dedicated program for the slave whose "Priority" is set to "Yes".