[TYPO3-english] unexpected value exception

[Tim Clark]

I'm getting an error on a typo3 4.2 installation:
$toolbarItem "workspaceSelector" must implement interface backend_toolbarItem

Is there a solution to repairing this? I believe the issue is caused by the version of PHP running on the server being upgraded? Would this be correct?

Is there such a thing as a straightforward. impossible to stuff up, step by step guide to upgrading a Typo3 4.2 installation?
I have a very old site and due to the version of PHP on the server being upgraded. Can no longer get into the backend,

Appreciate any advice you might have?