Hi!
VEINS is based on MiXiM. You can integrate VEINS directly with INET, there is no need to use MiXiM.
Regarding the error you posted -- there seems to be some debug output missing. The VEINS website actually has a pretty decent list of common errors and a short guide on how to get some basic debug information, which should help you along if you have some programming experience.
I've also compiled a list of additional errors I've encountered several times, though this is by no means a completed list (and I have about a dozen issues I still need to document), but maybe your problem appears there:
https://github.com/namnatulco/veins/blob/master/usagenotes.mdAlso, consider posting on the stackoverflow page if you have a problem related to programming, because that page has better quality control and allows you to easily edit your original question to add information like this :-).
Regards,
Rens