This was reported by a user on Mender Hub, read the discussion for full details.
Here are some extracts,
User has a trailing "/" in mender.conf
Which lead to this
Above produces 404 error. User reported that removing the trailing "/" fixes the problems they where experiencing.
FWIW your documentation has conflicting information in it (see https://docs.mender.io/1.7/client-configuration/configuration-file/configuration-options#servers 1). The ServerURL parameter is documented as to include a trailing slash. Yet the Servers parameter (array form) does not show/include one.