On another note (and as a bump) I'm also getting loads of "Translation not found" errors too, even though they are all defined in xlf files in 'app/Resources/translations/en-GB' and the locale is set to 'en-GB'. Seems this is an old version of symfony as I found out when I looked at the docs.....
As an update the categories show fine as options for the main menu now. No idea why they weren't - other than maybe some caching issue? The only outstanding problem is this "unserialize" one but I have have no idea where the data is coming from. Guess I'll just need to keep digging.