I have no idea. Yes, powered off/on the hangar. Used one SRV previously before the problem occurred. Repaired and returned to the hangar. Never used the second one. Even quit/restart the game and tried a planet landing in another another system. Back to the bubble, sold the hangar, quit/restart the game. Bought a 4G hangar and SRVs. Went to a planet and all is working OK.
On the way entered a Combat Aftermath Detected USS with nothing there. Two minutes later a wedding party of three ships shows up. The game is getting weird. Only recent play difference was my highly engineered Krait Mk II including the Fuel Scoop and Point Defence. Never engineered everything possible on a ship before.
Also noticed delay in updates. Most gathered mats Guardian or otherwise and mission rewards aren't immediately added to the inventory. My ISP is 240Mbps download so I'm pretty sure it is not me. As a side note items purchased at the Frontier Store are immediately available after payment. Previously it could take some time for them to arrive.
Note at a Guardian structure you have to know what you are doing with an SRV. Charging up the six pylons with lots of obstacles and rough terrain is a lot harder than a security base. Skimmers even fire missiles at you. Obstacles can protect the SRV while returning fire. Bring mats to repair, refuel and rearm your SRV. Program the turret toggle for fast switching as the turret view is used to shoot pretty much everything. I had it on a toggle switch but moved it to a pinky finger slide up switch on the Saitek X-56 grey HOTAS throttle for a faster response. Slide down reverses the ship thrusters and SRV reverse.
Regards