Upgrade to version 4 causes blank dashboard with German install

I have an odd problem that I wonder if somebody clever might be able to help me with.

I have a German install of wordpress hosted with greenwebhosting.ch using php version 5.3.11. I am stuck with the hosting account as it is a client’s.

The problem is upgrading to anything higher than wordpress 3.9. I am upgrading manually as the auto-updates fail, using the German install files, and although the upgrades seem to work as soon as I log out and log-in again I have a blank dashboard. I have tried upgrading to 4.1 and 4.2.2 and both times I have had to roll back to the latest 3.9 version. This time with 3.9.6 I am struggling to keep it stable. If I have the native dashboard plugin installed and work in the admin in English everything works absolutely fine. As soon as I change to German in the dashboard I get a blank screen, or if I log out and log in with German I get a blank admin. Even with native dashboard uninstalled and only the base German install the problem exists.

Once I have the blank screen I can deactivate plugins, which eventually allows me access again, however the plugins that seem to ’cause’ the problem change each time I test. I have even deactivated akismet which solved the problem, but then it is ok when reactivated and then another plugin will be the trigger.

I don’t really understand what is happening here. Is there a plugin that is causing this problem? I have cartpress installed, and a number of others I can provide a list if someone thinks it will help.

When I was troubleshooting the problem with the 4.1 upgrade, I came across threads that said there were problems with the way version 4 languages work with older versions of php. Does this sound like the problem, although I would have thought php 5.3 would have been ok.

I have installs of wordpress in other languages on other hosting servers and they all work ok, so I am at a loss as to why this one doesn’t work.

Any threads/advice anyone has to offer would be so helpful. Thanks.