glitch: strange (disappearing) error message

[Grendel]

Hi all,
I looked around in the forum, but I might not have used the correct keywords to find an answer...
I'm using Mambo 4.5.2 with the current SMF Forum 1.0.4 and the bridge. Both work fine, there's just a little glitch that might confuse no- or lowtech users: When logging in using the Mambo login field (there's no alternative, I'm trying to keep the whole site as simple as possible), for a second, the forum main site appears in the content field with red text (in German) stating that an error has occured. Below is a login form by SMF with my details (i.e. as administrator) already filled in. A second later, the page reloads and I get to the main page, logged in and everything is alright, I'm logged in both Mambo and SMF forum. What's the matter with the error message?
regards,

Grendel