I haven't used Pilit2ATC for a while and a friend mentioned that he couldn't get it to work so I fired it up and also checked for updates.  Found 2.6.20 and installed it hoping it would solve the issue of the previous version not connecting to X Plane 11.51.  But it didn't.  I'm running XPUIPC 2.0.48 and that was working before.  I have tried disabling all plugins in X Plane 11 and still Pilot2ATC will not connect.  I have checked my computer's IP address and made sure that is set as the server IP address in the xpuipc.ini file.  Is anyone else having this issue?

I just tried it and P2A connected with XP 11.51.

Thanks for the input.  I just tried Pilot2ATC with P3D V4.5 and v5.1 and it works with those.  I disabled all plugins for x plane and even tried two other versions of XPUIPC and no luck.

I tried uninstalling Pilot2ATC, deleted the folder and reinstalled and same issue.

So here is my Pilot2ATC log file  when I try to connect to X Plane 11.51.

Program StartedPilot 2 ATC (P2A) - version 2.6.2.0_x64  02/03/2021 12:17:53
Trying to create DB_Dyno.
DB_Dyno created successfully.
Init 15% done.
Trying to create DB_Base.
DB_Base created successfully.
Application frmP2AMain starting 02/03/2021 12:18:22
Loading Joystick Devices - 5 devices loaded.
Loading Form
Main form opened: Pilot2ATC 2020 v. 2.6.2.0_x64      Navigraph - AIRAC cycle: 1801
Loading Joystick Devices - 5 devices loaded.
Loading Joystick Devices - 5 devices loaded.
Loading Joystick Devices - 5 devices loaded.
Loading Joystick Devices - 5 devices loaded.
Joystick Device number 0 successfully activated.
Loading Map Control
LoadMapControl: panel 1
Creating ucMap1
SQLiteVersion: 3.8.11.1 | 2015-07-29 20:00:57 cf538e2783e468bbc25e7cb2a9ee64d3e0e80b2f | INTEROP_CODEC INTEROP_EXTENSION_FUNCTIONS INTEROP_VIRTUAL_TABLE NET_40 PRELOAD_NATIVE_LIBRARY THROW_ON_DISPOSED TRACE TRACE_PRELOAD TRACE_SHARED TRACE_WARNING USE_PREPARE_V2 WINDOWS
ucMap1 Created
ucMap DockFill set.
Setting Map Mode
Done Loading Map Control
Loading FPL Control
FPL Control Loaded
Loading Joystick Devices - 5 devices loaded.
Loading Joystick Devices - 5 devices loaded.
Loading Joystick Devices - 5 devices loaded.
Loading Joystick Devices - 5 devices loaded.
Joystick Device number 3 successfully activated.
Joystick Device number 0 successfully activated.
Joystick Device number 0 successfully activated.
Joystick Device number 0 successfully activated.
The SIM or FSUIPC/XPUIPC stopped functioning and P2A is Disconnected.
Error in frmP2AMain ConnectToSIM
FSUIPC.FSUIPCException: FSUIPC Error #9: FSUIPC_ERR_NOTOPEN. The connection to FSUIPC is not open.
   at FSUIPC.FSUIPCConnection.Process(Byte ClassInstance, IEnumerable`1 GroupNames)
   at P2A_2020.P2ASimulatorFSUIPC.Init()
   at P2A_2020.frmP2AMain.c6da96e2c3dc97ecb14521624b5c07984()
Application frmP2AMain ended normally 02/03/2021 12:19:33
Object of type SlimDX.DirectInput.Joystick was not disposed. Stack trace of object creation:
Object of type SlimDX.DirectInput.Joystick was not disposed. Stack trace of object creation:
Object of type SlimDX.DirectInput.Joystick was not disposed. Stack trace of object creation:
Object of type SlimDX.DirectInput.Joystick was not disposed. Stack trace of object creation:
Object of type SlimDX.DirectInput.Joystick was not disposed. Stack trace of object creation:
Total of 5 objects still alive.

So for some reason XPUIPC is not working with PIlot2ATC.  I have removed the XPUIPC plugin and reinstalled 2.0.48 but still no luck and it worked with this version before.

Well I fixed the issue.  I had to run Pilot2ATC as administrator.  Strange that is was working in P3D OK but not in X Plane 11.  But now it's working in both again.  Unfortunately when I uninstalled Pilot2ATC during my troubleshooting it removed the voices I had added so I had to reimport them back into the registry.  But everything is back as it should be.  A lesson learned.