PHP Error - Cannot modify header information - headers already sent
That error means that something is being output to the browser before it's supposed to be.  In this case, it's cookie information (or more specifically, the removal of a cookie by adding one with the same name, no value and an expired date).

The question is, what's calling removeCookie?  If you enable devMode, then the error should present with a stack trace where you can follow the programs execution to see what's leading up to that call.  My guess is something in the Import plugin (or maybe another plugin) is the culprit.