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
If you still are having problems, try this combination in catalog.cfg, the catalog configuration file:
SessionExpire 10 minutes
WideOpen Yes
The above setting will typically make MiniVend work when it is possible to work.