After extensive local testing, tonight I was hoping to start using Marketpress for real.
I uploaded a fully functioning customized Marketpress to my server and in the moment I activated the plugin, it shut down my permissions do access my Wordpress install (all users, not only mine). Just to be sure it was Marketpress, I deactivated all plugins via an FTP easy trick and activated only Marketpress. Same thing happened.
(I would REALLY appreciate if someone could shine me a light today - sunday - so I can meet my project milestone)