Okay, you have a bad PHP configuration. Think you have a lot of queries on your website.
Try to increase your memory_limit. Depends on allowed memory on your server. Try to delete unused modules on /modules directory. It probably will help.
Missing configuration class is strange, try to reupload it and set chmod according to Prestashop standards. However, your machine does not seem to efficient. But only try to reupload class if memory limit is increased on a huge value. Ask for server logs too, that is the most important, and post it safely here.