This is usually due to the HostnameLookups (Stronghold/Apache parameter) not matching for the two servers, secure and non-secure.
If it still does not work, then try changing some of the appropriate configuration parameters in minivend.cfg:
DomainTail No IpHead Yes
The above setting will typically make MiniVend work when it is possible to work.
WideOpen Yes
This completely disables IP checking.