[Python] OSError: exception: access violation reading 0x00000001

I try to communicate with a HV-Supply through a c dll with python. The most simple Function i get to work. But if i call the more complex function CAENHVInitSystem i get an Error: OSError: exception: access violation reading 0x00000001. I am quite new to ctypes in Python. As far as i know, this error maybe shows because some of my arguments have a wrong type. But how i can debug it more to know exactly which argument is wrong? does anyone see my mistake?

import os
from ctypes import *

bib = CDLL("CAENHVWrapper")

ret = bib.CAENHVLibSwRel() # This call works
print(c_char_p(ret))

sysType = c_int(1) #SY2527
link = c_int(0) #TCP/IP
#arg = c_char_p(b'149.217.10.241') #i change it for test to c_void_p but later the arg should be the ip adress
arg = c_void_p()
user = c_char_p(b'admin')
passwd = c_char_p(b'admin')
sysHndl = c_int()

# c function definition in the header file
#CAENHVLIB_API CAENHVRESULT CAENHV_InitSystem(
# CAENHV_SYSTEM_TYPE_t system,
# int LinkType,
# void *Arg,
# const char *UserName,
# const char *Passwd,
# int *handle);

# definition of the enum of the first argument
#typedef enum {
# SY1527 = 0,
# SY2527 = 1
#} CAENHV_SYSTEM_TYPE_t;

bib.CAENHVInitSystem.argtypes = [c_int, c_int, c_void_p, c_char_p, c_char_p, POINTER(c_int)]
ret = bib.CAENHVInitSystem(sysType, link, arg, user, passwd, byref(sysHndl))

print(ret)
print(bib.CAENHV_GetError(sysHndl))

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