I get the same error, but only on my local WAMP installation. It works fine on the webserver.
In the MODX error log a get the message
GuzzleHttp\Exception\RequestException sending GET repository for provider modx.com: cURL error 60: SSL certificate problem: unable to get local issuer certificate...
so I assume it’s because the local server can’t verify the SSL certificate.
I was able to fix it by downloading a certificate bundle from here and setting the openssl.cafile value in the “php.ini” to this bundle-file (similar to this stackoverflow topic).