The VBA COM line
Set Visum = CreateObject('VISUM.VISUM.180')
fails with the following error message:
'ActiveX component can't create object'.

The Python line
Visum = VisumPy.helpers.CreateVisum(...)
fails with an error message, containing at the end the string:
'Invalid class string'

The computer uses a 32 bit or 64 bit operation system.

PTV Visum needs to be registered again as COM server.

Until Visum 12 use an external tool in the start menu:
Register Visum as COM server (again):
Start -> All programs -> PTV Vision -> Visum -> Tools -> Register as COM server -> (Click right mouse button) -> Execute as Administrator.

Since Visum 11 an internal tool is available:
- Visum 11-12.5:
Extras -> Options -> (Working) Environment -> General -> Register as COM server.
- Visum 13:
Edit -> User Preferences -> Working environment -> Register as COM server.
- Since Visum 14:
Help -> Register as COM server.

Notes:
- Make sure to start PTV Visum with administrator credentials.
This procedure could be necessary again after the computer was rebooted.
- Check in the Windows register, which class ID is currently registered.

Example test scripts using early and late binding:
http://vision-traffic.ptvgroup.com/faq-files/PTV_COM_VBA_Instantiating_VissimVisumVisem.zip