Some Perch sites not loading following server migration/upgrade

Hello

I have 24 Perch sites.

Following a hosting provider server migration 20 still seem to work fine but four of them don't. On these the live site wont load beyond the first bit of Perch and if I try to sign in to Perch I get the white screen.

What the four sites that don't work seem to have in common is that they were all from around last summer and are the following versions: 2.5, 2.5.4 and 2.6. Older sites all seem to work fine.

Was there something obvious that changed around then within Perch that would give a clue as to the issue with the server migration so that my provider has a clue as to what to change?

An example of the four sites not working is www.fbhp.org.uk

Thanks - Mark