WordPress Upgrade - API files returning 404 on Multisite

Question

I've just upgraded to WordPress 5, and all is working fine on the main site.

However on the two sub-branch sites, when I try to create a new post, I receive a white screen and the console reports 404 errors when trying to retrieve the following files:

wp-includes/js/api-request.js?ver=5.0.3
wp-includes/js/dist/api-fetch.js?ver=2.2.7
wp-includes/js/wp-api.js?ver=5.0.3

The files are definitely there, all other files are being retrieved correctly from the same and similar locations.

There are no bugs reported in the debug file, I have reset permalinks and have tried deactivating all plugins.

Does anyone kindly have any suggestions?

If my question isn't welcome, it would be appreciated if it isn't downvoted - Stack Overflow is allegedly trying to become more welcoming - it seems this page hasn't caught on yet.

Answer 1

The error was caused by a rewrite rule in our web.config file:

<rule name="API" stopProcessing="true">
    <match url="API" />
    <conditions logicalGrouping="MatchAll" trackAllCaptures="false" />
</rule>