Log in problem

For some reason, blog is refusing to let me log in through the main login screen. It produces no error, merely cycles back to the login screen. Sometimes I can get in through edit-comments.php (it treats me as if I am logged in), other times trying this forwards me to the login screen. But if I persist with trying edit-comments, the second or third attempt generally works. Another of my blog authors is having the same trouble, so it must be a server-side issue.

I don’t know whether it can be related or not, but Bad Behavior has started refusing my comments also.

I’d be grateful for any suggestions.