The following two errors come up when a specific user tries to filter against the "Owner" field in Modern UX's Project Grid.
1. "The view could not be saved."
2. "CMN-0009: the search value '5018005' for the attribute 'Owner' is invalid."
It was determined that this specific user account did not have visibility to any Resource in the system.
Provide Resource View Access right at the very least, for the user to filter against this field. View right can be provided to particular Resource records, or to certain OBSs or Globally to all Resources.