Using the eZContentStaging extension, the 'check' feature (ping) on a feed returns a connection error with the following message:

Response received from server is not valid json. Decoding error: 4

The actual value of the stream returned by the target server is a Fatal PHP error with the following message:

Fatal error: Class 'ezcAuthenticationDatabaseInfo' not found in
<target-root>/kernel/private/rest/classes/auth/styles/basic_auth.php
on line 26

Since I know eZC is installed (through PEAR), and that the database_info.php file containing the ezcAuthenticationDatabaseInfo class is present, I assumed this to be a problem with autoloading & the PHP include path.

Reverting the following git commit 'fixed' the problem:

Change Composer use to avoid including legacy autoload.php when legacy folder is called 'ezpublish' (4d32bde)
https://github.com/ezsystems/ezpublish/commit/4d32bde856c03b3adcf5afbd8114fbb517e32d3f

I was unable to assert if this was a recent requirement change, or an actual issue.