Personally before I spent a lot of time trying to fix plugins (which is a good thing to do!), I would try to confirm if they're part of the problem. Have you tried disabling EVERYTHING (meaning all non-default applications/plugins) and also trying the default skin again?
That would help you figure out if the problem is in your core board or if it's something related to something added afterwards. If it happens with everything turned off and the default skin, the IPS support team would need to most likely take a look. If it suddenly starts working again, you know it's somewhere in the list of things you turned off earlier. So you can turn them back on one at a time until you find the culprit. (I would start with EVERYTHING off, and if working, start by enabling the theme again... then test. Then start with any applications, etc.)