My appointments system is currently not working for my

Hello,

My appointments system is currently not working for my users. It is loading forever or showing "500 internal server error".

It used to show an "out of memory error" so i increased my php memory limit (to 1024mb) and max execution time.

I'm on a VPS server, with 2gb ram and a quadcore processor... i guess i have enough ressources.

I also disabled all my plugins and tried to switch to another theme (still doesn't work).

How can I solve this?
Thank you !