LDAP authentication times out

We use LDAP to authenticate. It works fine, except after a few minutes (not sure exactly) I start getting popups that say:

“Authentication Required. This action requires you to confirm your password”.

Entering my password just gives an error “Failed to authenticate, try again”.
My only option is to logout and log back in, then everything works again for a few minutes.

Additional info.
This only seems to happen when editing “users” as an admin.

Can you check your logfile for errors? Does this happen on other computer/browsers as well?