Probably a configuration issue. It could be a problem with your .htaccess, but since you say the primary domain works, I'll guess it's the Sites configuration.
In /wp-admin/network/sites.php hover over the site, click Edit
On the Info tab, the Domain field should Not have a trailing slash.
for Subdomain Installation: the Path field should have a / (only)
for Subfolder Installation: should have the subfolder name with both leading and trailing slash, e.g. /salmon/
For either, you definitely should have a check mark by "Update siteurl and home as well."
After you have Saved Changes, the heading "Edit Site: " at the top of the screen should be followed by a working URL to visit the site.