JSON error on WP multisite

Issue Summary

When I install WP as a multisite (subdomain or subdirectory) and edit a post, it gives an error when trying to save the changes.
“The response is not a valid JSON response.”
There are several console errors logged, related to …/wp-includes/js/dist/vendor/wp-polyfill.min.js?ver=7.4.4

Troubleshooting Questions

It only happens for a multisite install, but for subdomain as well as for subdirectory.


I tried several times, also with fresh installs and new WP subsites.

System Details

Local version 5.10 and 5.10.1 on a Mac Big Sur 11.2.3 (and 11.2.2)
local-lightning.log (147.0 KB)

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