Admin not reachable on 25 different WP installs at different hosts

Hello,

We have a strange problem.

We have about 50 wordpress sites spread over about 6 or seven different hosts versions vary from 2.9 – the latest.

Since a few days we are not able to access the admin section on about half of the site. We either get a 500 internal error or a 404 not found.

Some older versions I updated manually through ftp this seemed to solve the problem but the next day it was back at the 500 error.

One of our hosting companies informed us of a server update prior to the problems so we figured it was the update. But also sites our other hosts are giving the same problem. Some sites weren’t touched or changed for months.

Helpt with this strange problem please