The resource is valid, but VISA cannot currently access it

I try to control a Keithley 2306 by using VISA (the goal is using it
from python).

After installing NIVISA541full.exe, syscfg1601full.exe and
NI4882_1500f0.exe I started NI MAX but I get following error:


Error opening resource:
USB0::0x3923::0x702A::010A65AB::RAW

VISA:  (Hex 0xBFFF0072) The resource is valid, but VISA cannot
currently access it.

Sending IDN? from VISA Test Panel gives only:

Write Operation (*IDN?\n)
No Error

The Windows device manager shows an NI-VISA USB Device: "Device Model
Name", status: working correctly.


Maybe interesting also:

C:\Windows\system32>py -m visa info
Machine Details:
   Platform ID:    Windows-7-6.1.7601-SP1
   Processor:      x86 Family 6 Model 42 Stepping 7, GenuineIntel

Python:
   Implementation: CPython
   Executable:     C:\Users\vos\AppData\Local\Programs\Python\Python36-32\python
.exe
   Version:        3.6.0
   Compiler:       MSC v.1900 32 bit (Intel)
   Bits:           32bit
   Build:          Dec 23 2016 07:18:10 (#v3.6.0:41df79263a11)
   Unicode:        UCS4

PyVISA Version: 1.8

Backends:
   ni:
      Version: 1.8 (bundled with PyVISA)
      #1: C:\Windows\system32\visa32.dll:
         found by: auto
         bitness: 32
         Vendor: National Instruments
         Impl. Version: 15728640
         Spec. Version: 5243904
      #2: C:\Windows\system32\visa32.dll:
         found by: auto
         bitness: 32
         Vendor: National Instruments
         Impl. Version: 15728640
         Spec. Version: 5243904


Any help would be appreciated!

Regards
Volker