Fix a Wordpress/PHP problem on our website

We have a hotel rooms reservation and car rental cooking website that was built during the past few months and was recently completed. The website is a pretty simple wodpress site, which retrieves XML data from a large hotel booking company and uses the worldpay gateway (through woocommerce) to bill clients. While he completed the coding work and brought the website to production level, the coder had to bail out of the project for personal reasons, leaving the website with an irritating error he hadn't had time to deal with. The website was operating properly until the coder tried to begin the final stage of the website optimization. Once this stage took place, and specifically, a cache plugin was installed, errors began to appear. ** The attached file is a text file that includes the lines of error that currently appears in the website's header at all times. While trying to understand the problem through communication with the website coder, he mentioned a few things that might help you understand the problem: 1. The problem appears after he installed and enabled cache on the website 2. He says there isn't a need for a change in the code itself because the problem is that it seems the website can't detect a core function (add_action) 3. Running a line by line debug should help reveal the source of the problem 4. Due to the fact that it's practically impossible for the website to detect core functions (as mentioned above), the website might be loading an HTML cache of some sort. He believes that we should eliminate any static files such as these, should they exist. 5. The coder had to reinstall the website 4 (!) times because every time he did so, and enabled the cache, the website got screwed up again. 6. Although he disabled and deleted the cache plugin, the problem remains, so he thinks there maybe an additional layer of cache he missed. 7. When he ran the website on his own local server, the website was running without any errors, even after enabling the cache (super cache) plugin, so he suspects it might be a server related problem. 8. The MySQL DB of the website is around 180MB which is relatively big for a simple website, but that is because the coder added a Log table in it. Now, these are all the coder's remarks. He might be right and he might be wrong. I though I would add these to give the winning freelancer a few directions of possible resolution of the problem. The hosting server is a shared one. We have several shared hosting plans in multiple companies. Should you come to a conclusion that the problem is server related and that we should move to another server, we would gladly give you the new server's credentials so you can migrate the website to the new server. The coder, while unable to help us out with this, had stated several times that once we find a freelancer to fix this problem, he would gladly answer any questions the freelancer might have regarding the site architecture or anything that might help us out. To summarize our requirements, we need you to: 1. Fix this problem on our website 2. Make sure it is functional again, including the XML retrievals and worldpay payment gateway 3. If needed, migrate our website to a new server 4. Once everything is working smoothly, we need you to be available for questions and small fixes (if any) for a month from the project completion date A very easy project for someone who knows what he's doing, and should take more than a couple of days. * We will hire only an experienced Wordpress specialist with solid PHP knowledge. * 2 Milestones. The first when problem is solved, and second after a period of beta testing (2 days at most). * No upfront payment before the job begins. To avoid auto bid postings/bots, please include the word HOTEL in your application for this project. Have a question? Please ask beforehand. We need a trustworthy and professional coder to replace our previous one. Do a good job and we'll definitely use your services again.