iThemes Security plugin - when the Filter Long URLs option (under system settings) is enabled, it breaks the One-Click Login feature. Also, when the iThemes Security plugin is installed on MainWP Dashbboard site, it will probalby break the Install by Upload feature because it blocks requets from child sites, so child sites can’t download plugin/theme from the dashboard that needs to be installed.
Regarding the Cerber Security, Antispam & Malware Scan, I have seen it multiple times causing the connection problem, @jandrwallace thanks for sharing the solution.
Earlier, I have seen Wordfence WAF causing problems too, but after a few WAF Rules updates, everything is working properly.