Update of my post from yesterday:
No debug messages, but (I think after running Network Update) now get redirect loops, and links on pages having unmapped domain name.
What are the meanings of the Domain Options settings, and what scenarios do you use them? I am using only "Permanent redirect" and "User domain mapping page".
New debug message:
Notice: Undefined index: path in /home/USERNAME/public_html/wp-content/mu-plugins/domain_mapping.php on line 481