I can say that I encountered the same issue as colageneral and found out how to resolve the issue.
In my situation, the problem was that I had forgotten to install the module via the admin tool. (Modules > Action Recorder)
What happens is that when the module is not installed and you try to send a message via the form on 'index_maintenance', the function that checks whether the module is installed will return false, leading to a default error message being generated. (in this case, the message 'Error: An enquiry has already been sent. Please try again in 15 minutes.')
Version info:
* OsCommerce v2.3.1
* Store Mode v1.3 (30/05/2011)