This is a hosting problem, from what we have seen in the forums.
This problem has been posted many times and i haven't seen any real explanation and solution, most probably because it is not related to Osclass.
But you should check what new you have installed, enable debug.log for Osclass and check for errors and also ask your hosting provider about it.
I see you have done several modifications also to the theme (and to Osclass maybe, i don't know), and you are also using Cloudflare (for no reason). So you have many things that could produce that error, and it impossible to tell.