Drupal website troubleshoot

I have a website which was done 3 years ago by a webdesigner which I no longer have contact with. I believe it was done in DRUPAL (not sure). Last week, I decided to transfer the website to another server. So I transferred the files and database and everything seemed to work fine. Today, I realized that some sections don't seem to be working. The website is https://newfacebrushes.com. If you try to create an account, or do the checkout you will receive an error (HTTP ERROR 500). Initially I thought it could be a database connection problem, but I can access it fine when doing the log in for the admin page ([login to view URL]). The website was working fine in the other server so I'm not sure about what is going on. After digging for a while, I realized all the pages that are having problems are related to plugins. In the back-end of WordPress, for example, you can have access to plugins, but the admin page I have access to only let me change page content. What you need to do is to bring the website back to full functionality. The only thing I can give you is access to the server through FTP as I don't have access to any drupal back-end or similar.