There are many possible causes for this error. Using MiniVend 3.08 and above, the main 3 are:
One possible cause is that you must define METHOD=POST in all of your forms, not METHOD=post. The lower case form is not recognized in the HTTP spec.
MiniVend 3.05 and later has a new TolerateGet directive which allows you to have forms with METHOD=GET. To use, set the session ID in your form
<INPUT TYPE=hidden NAME=mv_session_id VALUE="[data session id]">
then set TolerateGet Yes
in minivend.cfg (the main configuration file) and use
METHOD=GET on forms that have problems.
This may prevent errors due to one of the causes above.