The testing was on 64-bit Win10 Pro. Other RegFN/CallFN function calls seem to work as expected, just not that one.

Running the app in Win7 compatibility mode on Win10 works fine as well. That was an early workaround, but didn't want to have to tell clients the app isn't Win10 compatible ;)

It does seem to be the specific combination of using the FoxTools functions to call Win32API ReleaseDC(), on Win10 (64-bit at least, dunno about 32-bit).

>Too lazy to check up on my SWAG ;-)
>There was some 16-bit functionality kept in Foxtools from at least FPW2.6, probably FPD as well, and I believe RegFN / CallFN were among those.
>If that is true, perhaps they snipped more 16-bit backwards compatibility. Or You could try under a 32-bit version.
>Did you set compatibility with XP (dunno if that is still around in Win10)
>
>>This is a notice of a VFP incompatibility I recently found when run on Windows 10.
>>
>>Some code required calling Win32API functions to get the user's current Screen DPI settings. One way to declare and call the required functions is via the DECLARE - DLL command. One code sample of how to do this is at http://www.spacefold.com/colin/posts/2005/12-01FONTMETRICandScreenD.html .
>>
>>It is also possible to register, then call Windows API functions using the RegFN() (Register) and CallFN() (Call) functions of FoxTools.FLL. I inherited some code that used this technique. The following code works on Windows prior to version 10, but fails on Win10:
>>

>>LOCAL lnRelease
>>
>>* Register the Win32API ReleaseDC() function
>>* i.e. release the Device Context when finished with it:
>>m.lnRelease = RegFN( "ReleaseDC", "II", "I" )
>>
>>* Call the function:
>>=CallFN( m.lnRelease, 0, m.lnHDC )  && fails on Win10 with "DdeReq failed C 4006"
>>

>>The equivalent code using DECLARE - DLL is a bit simpler and easier to understand, and when declared and used that way the ReleaseDC() call does not fail on Win10.
>>
>>I'm not sure when DECLARE - DLL was added to the language, before then it may have been necessary to use the FoxTools functions. In any case the solution seems to be to convert to DECLARE - DLL, at least for Windows 10 compatibility.