We are suffering the same problem (on WP 4.8/Manta 2.6.1), and I’m not aware of any customisations to CSS.
“Failed to load content css: http://URL/wp-admin/admin-ajax.php/?action=dynamic_styles”
We have only “WP Statistics” and “Cryout Serious Theme Settings” plugins active, the second even though I don’t think it is needed. Inactivating these plugins has no effect.
I understand this may be a problem with commas in css, is there some code I could look in?