cant upload photos after move to new server php 5.4

Our ISP moved our site https://www.peteranne.it to a new server with php 5.4 instead of 5.2 and we can no longer upload photos to a directory with permission 755 (owner: read, write, execute). It worked fine bfore upgrade.
You see the photo bar going across as it tries to upload (50 kb photo BTW) then the bar goes red, you very briefly see HTML error and “0 photos uploaded”. Have asked ISP for details of HTML error but they haven’t yet responded, I think they are fixing lots of things that in their shoes I would have let my clients test first on mirror sandbox -;)

Any suggestions to fix/diagnose, please?

thanks Peter