Succession Plans - Exception during RestAction error when creating Incumbent Plan Type

Summary:

We get an Exception error when we create a Succession Plan of Incumbent type, and then we select an Incumbent

Content (please ensure you mask any confidential information):

Hello experts,

We are having an issue with Succession Plans: From My Client Groups → Succession Plans → Add → Selecting Incumbent as Plan Type and then selecting an Incumbent, we get this error "Exception during RestAction":

This only happens in Redwood and not in Responsive, and only happens to Custom Role.

Furthermore, this error is appearing from a few days, never occurred before so we don't know if it could possibly be related to new releases.