Issue http 500 error, upgraded my vps yersterday.

[stas styler]

Hello,
Yesterday I upgraded my vps with memory and storage.
I use plesk 17.5.
My contact forms on any website don't work.
it says:

This page isn’t working
mydomain.com is currently unable to handle this request.

HTTP ERROR 500

And I also have seen that sometimes I get error by plesk when I try to login to panel.
It says something about file manager, after refresh I can login.

can you please help me what should I do? or check?

 

[Bitpalast]

What are the error logs saying? Normally they mention the reason for an "internal server error" very specifically.