[Pro Sites] Login cycles back to login page

When a user logs out of a newly created pro site, then tries to log back in, the login page keeps cycling back to itself without logging the user in.

This is a fresh install of WordPress 4.9.9 multisite with Pro Sites, Domain Mapping, WPMU DEV Dashboard, and Snapshot.

Testing has shown that turning off domain mapping will eliminate this issue, even though there are no mapped domains. This is a sandbox site, so any changes are okay.