I have an account at StackOverflow (see http://stackoverflow.com/users/259059/floele). From there, I can see "network profiles" and see that I also have an account at StackExchange (see http://stackexchange.com/users/95029/floele).
Logging in to StackExchange (and thus also trying to re-login to StackOverflow) does not work however. In my StackOverflow profile, I can clearly see that the e-mail address "firstname.lastname@example.org" has been used for registration, but this account does not exist on StackExchange (in fact, I created the account just now so that I can post here at all).
So how can I login?