Filename Length, Reader 10 and XI

We have a rather motley combination of Reader installs on our network, from 9 through to DC (yes, I'm trying to get them all updated, but with 10,000 machines...).  This particular problem has be a little stumped.

There is a file on one user's machine (and I suspect others will have this as well) where the path and filename together exceed 255 characters. If this is opened with Reader 10, it opens OK. If we try with Reader XI, we get "Access Denied".  Same user account, same filename, same permissions on the file.

Was there any significant change between reader 10 and XI that could cause this to happen ??