No backend available

Yesterday the VNA example worked. Today I get the error:

C:\Users…\Documents\LimeVNA\pyLMS7002M-master\examples\VNA>python r50
Searching for LimeSDR… Traceback (most recent call last):
File “”, line 145, in
limeSDR = LimeSDR()
File “build\\egg\pyLMS7002M\”, line 49, in init
File “build\\egg\pyLMS7002M\”, line 146, in findLMS7002
File “build\\egg\pyLMS7002M\”, line 24, in findLMS7002
File “c:\python27\lib\site-packages\usb\”, line 846, in find
raise ValueError(‘No backend available’)
ValueError: No backend available

Q: What is a backend? Does this mean No USB driver?

I get the same error if I unplug the LimeSDR USB cable.

In attempt to fix the problem I tried to install:
C:\Users…\Documents\LimeVNA\pyLMS7002M-master\cyLimeLib\win>python install
Traceback (most recent call last):
File “”, line 16, in
from Cython.Distutils import build_ext
ImportError: No module named Cython.Distutils

Yesterday it worked fine, today I get the above errors; why?
Also I know nothing about python. However; it seems similar to Pascal and the type casting seems to work the same.
Any help is appreciated.

I’m not a Windows user, but I’m guessing it could be a PATH or PYTHONPATH environment variable issue, with these having being set up previously but not configured to be persistent across reboots etc. A quick search turned up:

Thank you andrewback, your reply may be close to answer my problem. While the PATH environment variable is “sticky”, in Windows, the PATH did change. The python PATH did not change; but, other variables disappeared. The PATH may have been altered when I installed the Smith Chart python application. The VNA example worked before the installation; but, not after. Even one of my own Pascal programs broke after this incident. Then again, it may be an entirely different cause. Maybe my spyware program got it; do not know. I did make a backup of my harddrive and also the PATH. I,ll try and restore the it.