I was curious if you had solved your problem with Authorize.net. I'm currently having the same problem with my shop. The last successful credit card went through around July 14th and since then its been bouncing back everything as declined but if I log into the Authorize.net control panel there is no record of the transaction at all (where there should be a declined card under unsettled transactions). I hadn't changed anything with my shop or installed any new mods around that time, so I was also at a loss as to why it would just suddenly stop working.
I also tried getting a new transaction key and reinstalling the mod.
After that didn't work, I called Authorize.net to see if anything had changed on their end, but they didn't have any idea what the problem might be and told me to call my credit card processor. I'm still waiting on a call back from them, but I can't really think of anything that it could be with the credit card processing company.
I also use Siteground and I've always gotten that same CPanel security certificate error. I didn't think to check and see if Siteground may have changed something that could be causing the problem, but I'll look into that today since we both use the same hosting and it stopped working at the same time.
But if you've managed to fix this, I'd really appreciate any help.