Miscellaneous Problems with Multisite

I’m having trouble once in a while getting an error like “Sorry, can’t access this site” when I do things on sub sites of a multisite installation. Most things work, like navigating posts, replies, posting, etc. Some plugins have problems.

The problems only happen on subdomain sites. The main site (no subdomain in the address bar) works fine.

I notice when I go to the dashboard for any subdomain, the console has these errors:

GET https://subdomain.domain.com/wp-content/plugins/bbpress/includes/admin/css/admin.css?ver=2.5.10-6063 
jquery-migrate.min.js?ver=1.4.1:2 JQMIGRATE: Migrate is installed, version 1.4.1
(index):206GET https://subdomain.domain.com/wp-content/plugins/powerpress/css/dashboard.css 
(index):129GET https://subdomain.domain.com/wp-content/plugins/powerpress/player.min.js 
(index):694GET https://subdomain.domain.com/wp-content/plugins/powerpress/images/itunes_modern.png 404 (Not Found)
(index):206GET https://subdomain.domain.com/wp-content/plugins/powerpress/css/dashboard.css 
itunes_modern.png:1 GET https://subdomain.domain.com/wp-content/plugins/powerpress/images/itunes_modern.png 404 (Not Found)

The main domain only gets this in the console.

jquery-migrate.min.js?ver=1.4.1:2 JQMIGRATE: Migrate is installed, version 1.4.1