Installer doesn't work without rewrites
The multisite code added to the base ses url in the module config removed the index.cfm so you need rewrites to be able to install.
The commit with the offending code is here https://github.com/Ortus-Solutions/ContentBox/commit/87345190f5930a6228fe62c1d58c4eb5d0f51c6c
To prevent users from having to edit two places in their code to enable/disable rewrites in addition to needing to edit a core file that might get overwritten on upgrade, I would recommend making the interceptor smart enough to check out the base URL setting and see if there's an index.cfm in there. That will allow what's in the routes.cfm to still drive the inclusion or exclusion of index.cfm for the whole app.