I’m getting this error when I use this extra command line “-f JSON:127.0.0.1”. But this command was runing ok previously. Anyone know how to use a older SITL version for a simulation ?
thank you men work again for me 
I still have the same issue for trying the swarm simulation
After I press the “Copter Swarm - Single Link” and “Copter Swarm - Multilink”
A pop up window saying “Failed to connect to SITL instance”
The simulation is work fine with a single vehicle settings
Here is my log
CustomMessageBox.Show
CustomMessageBox thread calling Base Thread
CustomMessageBox thread running Base Thread
INFO MissionPlanner.Utilities.Download - Get https://firmware.ardupilot.org/Tools/MissionPlanner/sitl/CopterStable/ArduCopter.elf
INFO MissionPlanner.Utilities.Download - https://firmware.ardupilot.org/Tools/MissionPlanner/sitl/CopterStable/cygatomic-1.dll
INFO MissionPlanner.Utilities.Download - https://firmware.ardupilot.org/Tools/MissionPlanner/sitl/CopterStable/cygquadmath-0.dll
INFO MissionPlanner.Utilities.Download - OK
INFO MissionPlanner.Utilities.Download - got LastModified C:\Users\user\Documents\Mission Planner\sitl\cygatomic-1.dll 2/12/2022 9:35:24 vs 24/4/2024 4:02:19
INFO MissionPlanner.Utilities.Download - https://firmware.ardupilot.org/Tools/MissionPlanner/sitl/CopterStable/cyggcc_s-1.dll
INFO MissionPlanner.Utilities.Download - OK
INFO MissionPlanner.Utilities.Download - got LastModified C:\Users\user\Documents\Mission Planner\sitl\cygquadmath-0.dll 2/12/2022 9:35:24 vs 24/4/2024 4:02:19
INFO MissionPlanner.Utilities.Download - https://firmware.ardupilot.org/Tools/MissionPlanner/sitl/CopterStable/cygssp-0.dll
INFO MissionPlanner.Utilities.Download - OK
INFO MissionPlanner.Utilities.Download - got LastModified C:\Users\user\Documents\Mission Planner\sitl\cyggcc_s-1.dll 2/12/2022 9:35:24 vs 24/4/2024 4:02:20
INFO MissionPlanner.Utilities.Download - https://firmware.ardupilot.org/Tools/MissionPlanner/sitl/CopterStable/cyggcc_s-seh-1.dll
INFO MissionPlanner.Utilities.Download - OK
INFO MissionPlanner.Utilities.Download - got LastModified C:\Users\user\Documents\Mission Planner\sitl\cygssp-0.dll 2/12/2022 9:35:24 vs 24/4/2024 4:02:20
INFO MissionPlanner.Utilities.Download - https://firmware.ardupilot.org/Tools/MissionPlanner/sitl/CopterStable/cygstdc++-6.dll
INFO MissionPlanner.Utilities.Download - OK
INFO MissionPlanner.Utilities.Download - got LastModified C:\Users\user\Documents\Mission Planner\sitl\cyggcc_s-seh-1.dll 8/4/2024 11:59:10 vs 24/4/2024 4:12:18
INFO MissionPlanner.Utilities.Download - https://firmware.ardupilot.org/Tools/MissionPlanner/sitl/CopterStable/cyggomp-1.dll
INFO MissionPlanner.Utilities.Download - OK
INFO MissionPlanner.Utilities.Download - got LastModified C:\Users\user\Documents\Mission Planner\sitl\cygstdc++-6.dll 8/4/2024 11:59:12 vs 24/4/2024 4:12:20
INFO MissionPlanner.Utilities.Download - https://firmware.ardupilot.org/Tools/MissionPlanner/sitl/CopterStable/cygwin1.dll
INFO MissionPlanner.Utilities.Download - OK
INFO MissionPlanner.Utilities.Download - got LastModified C:\Users\user\Documents\Mission Planner\sitl\cyggomp-1.dll 2/12/2022 9:35:24 vs 24/4/2024 4:02:23
INFO MissionPlanner.Utilities.Download - OK
INFO MissionPlanner.Utilities.Download - got LastModified C:\Users\user\Documents\Mission Planner\sitl\cygwin1.dll 8/4/2024 11:59:12 vs 24/4/2024 4:12:24
INFO MissionPlanner.Utilities.Download - Get https://raw.githubusercontent.com/ArduPilot/ardupilot/master/Tools/autotest/sim_vehicle.py
INFO MissionPlanner.Utilities.Download - https://raw.githubusercontent.com/ArduPilot/ardupilot/master/Tools/autotest/sim_vehicle.py OK
INFO MissionPlanner.Utilities.Download - https://raw.githubusercontent.com/ArduPilot/ardupilot/master/Tools/autotest/sim_vehicle.py got LastModified C:\Users\user\Documents\Mission Planner\sitl\sim_vehicle.py vs 24/4/2024 4:02:27
INFO MissionPlanner.Utilities.Download - Get https://firmware.ardupilot.org/Tools/MissionPlanner/vehicleinfo.py
INFO MissionPlanner.Utilities.Download - https://firmware.ardupilot.org/Tools/MissionPlanner/vehicleinfo.py OK
INFO MissionPlanner.Utilities.Download - Infinity bps 1024 0s 5.999883% of 17067
INFO MissionPlanner.Utilities.Download - Got https://firmware.ardupilot.org/Tools/MissionPlanner/vehicleinfo.py 17067
INFO MissionPlanner.Utilities.Download - Get https://raw.githubusercontent.com/ArduPilot/ardupilot/master/Tools/autotest/default_params/copter.parm
INFO MissionPlanner.Utilities.Download - https://raw.githubusercontent.com/ArduPilot/ardupilot/master/Tools/autotest/default_params/copter.parm OK
INFO MissionPlanner.Utilities.Download - https://raw.githubusercontent.com/ArduPilot/ardupilot/master/Tools/autotest/default_params/copter.parm got LastModified C:\Users\user\Documents\Mission Planner\sitl\default_params/copter.parm vs 24/4/2024 4:02:28
INFO MissionPlanner.Utilities.Download - https://firmware.ardupilot.org/Tools/MissionPlanner/sitl/CopterStable/ArduCopter.elf OK
INFO MissionPlanner.Utilities.Download - https://firmware.ardupilot.org/Tools/MissionPlanner/sitl/CopterStable/ArduCopter.elf got LastModified C:\Users\user\Documents\Mission Planner\sitl\ArduCopter.exe 8/4/2024 3:59:08 +00:00 vs 24/4/2024 4:11:48
Open MissionPlanner.GCSViews.FlightData FlightData
INFO MissionPlanner.GCSViews.FlightData - Activate Called
INFO MissionPlanner.Controls.HUD - OnResize start
INFO MissionPlanner.Controls.HUD - OnResize doing
I’ve had the same problem last week with swarm simulation on three different Notebooks/PCs. Single UAV worked perfectly but it was not possible to load three rovers with “Rover Swarm-Multilink”. I’ve got the same error message like you.
After updating MP to the last BETA version (from the HELP-tab in MP) swarm simulation worked perfectly.
BR Ingo