Turn on/off strereo mode -> crash in nvwgf2um.dll

To existing educational program, which passively use stereo mode (in automatic mode), I made “a plugin” which allow to turn stereo mode on/off (to allow presentation to more users than we have glasses)

All init code is: (error checking removed)
NvAPI_Initialize();
NvAPI_Stereo_CreateHandleFromIUnknown((IUnknown*)device, &myStereoHandle);
where device is typed as (void*) and contains (ID3D11Device*)
Application start with active stereo mode (as 3d is enabled from nVidia control panel).

To turn off stereo mode I use:
NvAPI_Stereo_Deactivate(myStereoHandle);

To turn on again I use:
NvAPI_Stereo_Activate(myStereoHandle);

It works, and I’m able to deactivate it and activate it again, but only few times.
If I made more activate/deactivate cycles, it crashes in nvwgf2um.dll. Sometimes after more tries, sometime even after third try.

At drivers 320.49 (Whql) it crashed:
Unhandled exception at 0x67E9A105 (nvwgf2um.dll) in aaa.exe: 0xC0000005: Access violation reading location 0x3F800000.
with cs:
ntdll.dll!77a015de() Unknown
[Frames below may be incorrect and/or missing, no symbols loaded for ntdll.dll]
[External Code]

nvwgf2um.dll!67e7db36() Unknown
nvwgf2um.dll!67fc6785() Unknown
AcXtrnal.dll!68a99480() Unknown
nvwgf2um.dll!684e1ddc() Unknown
nvwgf2um.dll!67fe7ab6() Unknown
nvwgf2um.dll!67eba71d() Unknown
nvwgf2um.dll!67e6a6a9() Unknown
AcXtrnal.dll!68a99480() Unknown
nvwgf2um.dll!67e3dfe9() Unknown
nvwgf2um.dll!67e3b6dc() Unknown
nvwgf2um.dll!67e38569() Unknown
nvwgf2um.dll!67f057f8() Unknown
nvwgf2um.dll!684e4f55() Unknown
nvwgf2um.dll!684e4fdf() Unknown
[External Code]

on latest drivers (326.41) it behave similarly (maybe crashed after longer time / after more activate/deactivate cycles)
Unhandled exception at 0x64CD375F (nvwgf2um.dll) in aaa.exe: 0xC0000005: Access violation reading location 0x00000226.
cs:
ntdll.dll!77a015de() Unknown
[Frames below may be incorrect and/or missing, no symbols loaded for ntdll.dll]
ntdll.dll!77a015de() Unknown
ntdll.dll!779f014e() Unknown
AcXtrnal.dll!73819480() Unknown
AcXtrnal.dll!73819480() Unknown
AcXtrnal.dll!73819480() Unknown
nvwgf2um.dll!653933ba() Unknown
nvwgf2um.dll!64cd3737() Unknown
AcXtrnal.dll!73819480() Unknown
nvwgf2um.dll!653933ba() Unknown
nvwgf2um.dll!64d047f2() Unknown
nvwgf2um.dll!64cd2593() Unknown
nvwgf2um.dll!64cc5b05() Unknown
nvwgf2um.dll!64cc03fe() Unknown
nvwgf2um.dll!64cbe105() Unknown
nvwgf2um.dll!64d964e5() Unknown
nvwgf2um.dll!65396c20() Unknown
nvwgf2um.dll!65396d48() Unknown
kernel32.dll!76b433aa() Unknown
ntdll.dll!77a19ef2() Unknown
ntdll.dll!77a19ec5() Unknown

Is anything what can I do with it?
E.g. Need some long delay between switches, or call some function to reset something?

Thank you very much for any info.