OK, so if no config has changed on the store, my guess would be either a config on the authorize.Net setting or in the network connect/firewall of your server?
I never use this gateway and it was oliver hine who originally build it, with only a little help from me for the integration, so I don't really know much about it. But things like this can be very awkward to solve. Also ask Authorise,Net if any upgrade to their system could have caused it? From there I think your options are limited to creating a new site on a new server and testing the gateway on that to see if you have the same problem or debugging the code.
I never use this gateway and it was oliver hine who originally build it, with only a little help from me for the integration, so I don't really know much about it. But things like this can be very awkward to solve. Also ask Authorise,Net if any upgrade to their system could have caused it? From there I think your options are limited to creating a new site on a new server and testing the gateway on that to see if you have the same problem or debugging the code.