You may find red a message in your ☰→Smart Lists indicating that your smart list is invalid or is being used invalidly. Below you will find all the possible reasons why:
invalid usage detected: cannot use !Section filter in custom section smart lists
This message appears when you use an advanced smart list filter like "!Morning", and then try to use this smart list in a custom section smart list. This filter means "match tasks in section Morning". So then to check which tasks are in this section we look at which tasks match this filter, bringing us right back where we started. This is an infinite loop!
When you attach a smart list to one or more sections, Marvin will try to automatically assign a section to a task (by looking for the first section with a smart list that matches). But you are free to manually assign tasks to any section regardless of whether it has an attached smart list. This means that the !Section advanced smart list filter is still a useful feature when using custom sections -- just not when attached to a section.
How to fix? Remove the !Section filter from this smart list's advanced filter, or remove this smart list from the section in your Custom Sections strategy settings.