wp-admin gets 404 on new subdirectory site

Hi there,

I've just moved a site from wordpress to Multisite, and after a few little hiccups it's all good.

So now I've added on a subdirectory site, /members/, and I can't get into it.

There was a previous install of wordpress at /members/ with not much in it. Using fantastico, I deleted it before moving to multisite.

I've tried browsing to domain/members/wp-admin and get a very weird 404 page - it's just text, no template etc.

I've tried going through network admin > sites > visit - same deal
also > dashboard, same deal.

I'm not sure where to even start troubleshooting this - have searched the forum but I'm not sure what I'm searching for. I've been using WP for a few years and am pretty confident I know my way around it quite well, but MS is a mystery.... I'd appreciate any light anyone can shed?