memory size exhausted

So this all started with a complaint from our host, Linode that our server's CPU usage is high and its affecting other resources on their machine. Our site went too slow, it used to take at least a minute to load. We tried to diagnose the issue but no improvement. So we created a new instance...

My site was displaying the following error. I thought it might've been a bug in the software so I attempted to upgrade to 1.5.4 from 1.5.3, but now the error happens halfway through the upgrade, and I can no longer access the XenForo Admin CP. The website now displays as if it's closed for...