I updated to 3.5 and suddenly all my category pages show 404 errors.
I tried to fix it by following the master list - disabling all plugins and changing from my custom theme to twentytwelve.
But the error still happens. I've tried several permalink settings - but the only one that works is "default" which changes the categories urls to manchesterfoe.org.uk/?cat=7 instead of manchesterfoe.org.uk/campaigns/climate. This means all the external urls to our site are broken if we leave that on.
The site is in maintenance mode, and as a temporary thing we'll try and set up a few redirects...
As an added bonus, our server admin has just gone to the Alps for a weeks holiday! :(