Configuration Page Unavailable

Problem

The configuration page is returning a "Site temporarily unavailable" screen or a screen with no theme and none of the configuration options (see below).
This happens after successfully adding at least one mailing list ("address book"). Second error page:

You are here

Home > Administration > Configuration
Dotmailer
Add to Default shortcuts

Steps to reproduce

Go to /admin/config/dotmailer and try to add more than two lists as available address books.

Proposed resolution

Check whether adding multiple address books is slowing the module down as well. This is the same issue previously reported, but now registry rebuild w/ cache clear is not fixing the problem.

Remaining tasks

User interface changes

API changes

Data model changes