400 Bad Request errors when logged in

I run a website on WordPress which permits users to sign in with their own accounts. However some users – including me – are experiencing intermittent 400 Bad Request errors. It happens quire frequently for me as I have to log in to edit the site. It’s wasting a lot of time as I have to clear my cookies and log back in to continue working. Plus I’m concerned it’s reducing the amount of visits my site receives.

I’ve tried deactivating plugins but this hasn’t worked. I’m struggling to troubleshoot this problem and would appreciate any recommendations where to start.