Currently Cuba uses version of Google Maps V3 API which doesn’t require free key at all (quotas for maps with no free api key are the same as for maps with free api key). So I doubt that you problem is key related.
If your maps work on local machine and not on the server (with the same settings and same version of Cuba) then I would be certain that the problem is on your server side.
do warnings still appear in the javascript console?
Note that API key property has been renamed, you have to set API key using property charts.map.apiKey now.
You should specify Browser key in there, unless you have paid subscription to Google Maps, in that case you should specify client Id using charts.map.clientId application property.