Activating tulip passport (GPIB) in MAX makes serial buffer flush fail?

Hi,

I apologize for my english, it is not my native language.

My situation:
I use labview, and I want to measure voltage using a HP 34401A Multimeter with GPIB (using a agilent USB-GPIB adapter). Furthermore, I want to control a motor using the RS232 interface (ISEL motor control unit).
Therefore, I use the RS232 of the computer (on the Mainboard). Both tasks were developed separatly and work quite fine independly. My task is to join both parts and thats were I have the problems.


Problem:
Case 1:
In MAX, I activate the serial (NiViAsrl.dll) VISA passport (deactivate all others)
-> In Labview: all serial SubVI work fine.

Case 2:
In MAX, I activate the GPIB (NiVisaTulip.dll) VISA passport (deactivate all others)
-> In Labview: all GPIB SubVI work fine.

Case 3:
In MAX: I activate both at the same time
-> In Labview: the GPIB works well, but the serial SubVI not. The "VISA I/O Flush" VI says Error -1073807299 (invalid buffer mask).
Here's a link to that error: http://digital.ni.com/public.nsf/allkb/AD03F1520934D67F86256D4B001DC2CE
 
Of course I checked the mask and it is like it should be (16). And it is working again as soon as I deactivate the Tulip passport in MAX.


More Informations:
- It is only related to this GPIB passport in MAX, the GPIB device doesn't even has to be connected.
- The settings of the serial connection are default and as specified for the motor control unit.
- The effect also occures in minimal examples (init the device and directly try to flush)
- I tried a different computer with the setup and it fails too (same software involved, including any passports/drivers).
- I tried to reinstall labview and passports/driver, still the same.
- The Computer: Its a 64bit windows 7 enterprise, but a 32bit Labview 2014.


My guess:
The tulip passport messes around with the buffers and I am failing to find the option to change that.

I tried to search the forum as well as the internet but I wasn't able to find a solution. Any Ideas?

Best regards
SteKo