My b2evolution Version: 1.9.x

I am building a site for Bury Gymnastics club (www.burygymnastics.org.uk). My idea is that I use the first blog in the root of the site as a site building app, where prmary administrators can edit the main site. a second blog is then set up in the /blogs directory of the site, so that editorial rights can be given to a different set of users without risk of corrupting the main site. In principle, it works fine, but I have a problem with the login on the /bl;ogs installation.

I do not seem to be able to log into the admin page on the second blog. It rejects the password which I know to be right.
if I request to change password, I get 'You have to use the same session (by means of your session cookie) as when you have requested the action.' error

I am sure it is a cookie problem, as , if I delete all the cookies, then I can log in. As I cannot very well ask users to do this every time they log in, can anyone else suggest a cure? Any help would be greatly appreciated!