Recently updated my Cygwin install on Windows 7 Ultimate x64 Workstation.

Tried to run startxdmcp.bat and got a Cygwin X error - failed to activate core devices

Looking in /var/log/XWin.0.log showed nothing obvious as to why the X server failed to start.

Ran Cygwin Setup.exe to update it Cygwin and got error:

The procedure entry point rl_filename_rewrite_hook could not be located in the dynamic link library cygreadline7.dll

cygwin_error

The fix was re-start Cygwin's Setup.exe and choose to reinstall libreadline7 6.1.2-2

The startxdmcp.bat command then worked as expected spawning a local X server and connecting to the remote Linux Servers display via xdmcp.

This could have been caused by a recent update that, from memory, gave an error stating that Cygwin couldn't replace some files that were in use so perhaps the upgrade failed and left things inconsistant.