error code 502 and 500 when accessing backend


It happens every now and then. After the error comes I refresh it (F5) and can work again.

it is happening with this site
I would be grateful if someone can give me a hand on it.

till last week I got only the error code 502 and som een blanco page but after refreshing it was "normal"
Following advies from someone else I have deactivated and deleted W3 total cache.
Now I am getting a new code error...


Oh ja, support access is granted.