I am trying to use a WP.org plugin called WP Maintenance Mode (https://wordpress.org/plugins/wp-maintenance-mode/). One of the features is that the administrator can still access the site front end while the site is showing the splash page to all other visitors.
If I have the WPMU Domain Mapping plugin activated on my network, however, that piece of functionality ceases to work and all visitors end up at the splash page, including the administrator. It doesn't appear to be tied to actually having a domain mapped either. If the Domain Mapping plugin is just activated, the issue occurs.
I realize that you do not support this other plugin, but I am wondering if someone can tell me what is conflicting between the two. The maintenance mode plugin appears to be pretty basic for the most part and nothing stands out as a potential issue when I read through the code, but I figured maybe someone with a better understanding of the Domain Mapping plugin could help.