Hi,
I first came across this error on a clients site.
They would get a 404 error when trying to edit or create an article or SP Pagebuilder page.
Upon doing some tests it appeared to happen when css / style / span was added to the text.
For example: <p><span style="font-weight: 400;">Pre-test</span><span style="font-weight: 400;"><br /></span></p>
This code would generate an 404 error.
I did a fresh install of the template (indigo_quickstart_j4_v2.1.0.zip) on another directory and repeated the test.
Same error!
There is something wrong with the template.