open_basedir restriction in effect (but that’s not the real problem, is it?)

I’ve just moved servers, and one of my three WP blogs works absolutely fine, but the other two are giving me this warning at the top of the page:

Warning: file_exists() [function.file-exists]: open_basedir restriction in effect.

https://oolong.co.uk/ac – the one that works – has a pretty similar setup to https://oolong.co.uk/oo (same theme) but the latter has a couple of extra plugins and modifications to the theme. The third blog, https://oolong.co.uk/zoo is based on the default theme, I guess, but heavily customised. There are no plugins that the two broken ones share but the other doesn’t.

Anyone got any clues at all?