I think I have found a bug.
I got a handful of these error messages below before 251,000 tons of enemy ground forces just appear on luna, no troop transports or anything in system (enemy being the new spoiler).
https://prnt.sc/5bgPGFIZkksb
There is some time, like a day or two, between these error messages and the troops appearing on luna.
Have I missed something or am I right in thinking the errors are somehow linked to the enemy transports not showing up?
Is there something I can do in SM to get rid of them?
TN start
39 years into game
one off error
Have a back up of the database from around a day before this if needed
v2.10 game
The exact same thing happened to me as well. Only difference is that I'm 13 years into game, and enemy ground forces are smaller.
Same error message, then couple days later enemy appears out of thin air on Luna.
Replying here as a reminder to myself for when I get home in about 10 days 
I checked the error when this was first reported and it was in the Aether Gate Transit function. I am now wondering if the raider troop transport had orders of transit into sol and then unload on Luna (for example), but failed the first order. If the code doesn't make the right checks, the transport might go to the coordinates that match Luna in its home system and then unload, with Luna still as the target. I'll check this theory when I return.
Something like this scenario just happened in my current game, so I am attaching the save in case it can help track down the problem.
Situation, see screeny "001 location bug 01.jpg", the ship/fleet "EX-02 Cosmic River Expedition" is in the middle of nowhere in the Lalande 21185 system adjoining Sol having gone into overhaul in that spot. The orders history list matching the current date show it thinks it has entered Sol and gone to Earth, refuelled resupplied and entered overhaul. The funny thing is it has got 100% fuel and supplies and I know it was at about 57% fuel when I gave the orders, so it has refuelled and resupplied as if at Earth but is for some reason in the wrong location on the map in the screenshot in a different system but the same angle and distance when measured from the Lalande star as Earth is from Sol. The planet in the Lalande system cannot refuel and resupply so it did not refuel there.
When I sent the ship to Earth there was a problem with autoroute, it would not see Sol, I had to give the transit orders stepwise by hand. Autoroute should see Sol because the ship class has a jump drive.
The order list shows 'squadron transit by subfleet' as the transit order for Sol transit. I normally use standard transit. I dont know if that was my doing, I may have misclicked the order but can't be sure. Either way it should not have ended up in the wrong location in OV.
Curiously the orders completed event notification gives the correct position for the fleet in Lalande.
If I close the game and reload the ship loads in the same location. I am going to leave it there and see how it goes, as it clearly thinks it is overhauling.
Just struck me as a very similar situation to the one you postulated Steve.
P.S. Another observation, in this save there is a jumpship in 93 Eridani 'Cosmic River' which cannot see Sol either. It does not appear to be able to see past the jump point which was discovered in Lalande which was added to Alpha Fornacis, as all the other JPs in Alpha Fornacis had been stabilised as part of an NPR gate network. However a jumpship which entered the Alpha Fornacis network recently 'Cosmic River Navigator' can see Sol in autoroute. It may be something to do with timing pre/post some other event as the ships which cannot see Sol have been deployed a long time. The ship which can see Sol from within the network redeployed after OV only recently. I mention this as I am not sure if this autoroute buglet was contributory to the location bug. I will try to test this.
