This isn't an error message, but nuclearslurpie mentioned that this is likely unintended behavior and a bug, so I am posting hit here to hopefully be addressed:
Behavior of escorting fleets using Formation Orders:
Rules as derived through experimentation:
1.) Escorts must be in their own fleet. You assign an 'anchor fleet' to the escorting fleet, and a distance and bearing.
2.) Bearing is relative to the toggles of Specific Threat, Nearest Hostile Warship/Contact, or Anchor Fleet destination
3.) When multiple toggles are selected, priority given to higher ones on the list.
For example, if distance is set to 1mkm, bearing is set to 0, and Nearest Hostile Contact and Use Anchor Destination are both set to yes, the escorting fleet will attempt to position itself 1mkm out from the anchor fleet first in the direction of the nearest hostile contact then, if no hostile contacts are known, 1mkm out toward the anchor fleets current movement destination.
The problem is that if the anchor fleet is not moving, and a hostile target isn't visible, the escorts return to 0 distance from the anchor fleet.
This means, if you want to have sensor escorts that form a ring of detection around your main fleet to take advantage of the efficiency of smaller sensors, it will only work if you already see hostile craft (which means you don't need the sensor escorts). If you are not moving and do not already have hostile contacts (such as using a carrier group that doesn't want to close on potential targets when you first jump into a potentially hostile system), the escorts will stay at distance 0 defeating the purpose of using a sensor net instead of one massive large passive in the parent fleet.
TL;DR The system as-is will work when moving your main fleet, but once you have your fleet where you want them, you have no ability to maintain the escort unless already detecting hostiles.
Current Jury-rigged solution: Fleets following Formation Orders and 'Use Anchor Fleet Destination' will maintain their bearing and range as long as the game sees the anchor fleet as moving. A follow command is considered movement, even if no distance was traveled during the last pulse. This works whether you are following another fleet or a planet or a DSP. You can emulate desired behavior (escorts always maintain distance and bearing, even when stationary) by having the parent fleet issue a 0 distance follow order on a stationary object. Currently, the best way I've determined this to work is to create a temporary Deep Space Population where I want my parent fleet to situate (You can't issue a follow order on a waypoint). This adds additional micromanagement whereby I need to clean up DSPs after combat.
The tactical downside of this is that bearing is relative to the target destination while moving, but then reverts to Bearing 0 = 'Due East' once the anchor fleet arrives at the DSP. This can create the unlikely but potential situation where an anchor fleet traveling 'due west' on the tactical map arrives at its destination, whereby all escorting fleets now switch sides relative to the anchor fleet (as Bearing 0 is now 'due east' instead of 'due west') and during this transition time, where escorts are out of position, hostile contacts stumble on the anchor fleet.
Proposed Solutions:
1.) When none of the toggles are set to Yes, Desired Bearing Offset is relative to cardinal directions on the tactical map. For example, if I have 4 escorts set to 0, 90, 180 and 270 bearing offset, I'll have escorts at the four cardinal directions, where 'up' is 'north' on the map. Currently, the game defines Bearing 0 as "due east" on the map when not relative to a specific target. This will allow for sensor nets that do not change their position relative to the anchor fleet. This is important, as a sensor net needs coverage, and moving to keep position relative to contacts can create situations where ships are busy redeploying rather than maintaining coverage if, for example, a hostile warship on the opposite side of the fleet suddenly becomes the closest contact, all escorts will redeploy relative to that new target, creating sensor gaps.
2.) Additionally, when none of the toggled options are true, or no options are toggled, escorting fleets maintain their desired distance. Thus, stationary anchor fleets who do not currently see hostile targets are capable of maintaining their escorts' positions.
2a.) If current functionality is desired (such as having non combat craft stay with the fleet during normal times, but run away from the closest hostile when they are found) implement a toggle such as "Always Maintain Distance" which will allow conversion between behaviors
