I had an issue very similar to apvandam post above, though I didn't get a 500 error but instead it took 5 minutes to load certain pages via certain routes to that page. I noticed in my CPanel error log (not prestashop log) that when this issue occurred, it could not find a folder under the src/someSubDirectories/ (which I no longer remember unfortunately). When i searched EVERY file under my shop for the folder name string, it didn't show up, except in a few compiled cache files. To keep it short, these findings led me down a long road that made me realize the folder name was generated during compile based on the hash of a password (or something similar to that).
I then realized, that during testing phase, I had previously created a CUSTOMER (ps_customer table) account with the same email as the ADMIN user account email (ps_employee table). I simply deleted that CUSTOMER user account via the BO and all my issues magically disappeared! Not sure if deleting any one customer account would yield the same results. Also, I had this duplicated email up and running for 3 months before any BO issues actually started showing up. Clearing all caches (manually and via BO) server side and browser side did not have any affect.
Anyways, thought I would share my solution since the errors being described are manifesting in many ways and my solution may work for you.
Cheers.:-)
p.s. Does any one know where to change my settings, so that my First and Last name do not show up in a Post? I would just like to display my handle (username) and location. I have no idea where the forum software is grabbing my first and last name from, I have searched every setting I could find.