This issue is caused when a user has cookies from other sites blocked in his browser, in case of subcontracting between two XTM instances in different domains this has to be allowed. Please go to chrome://settings/content/cookies?search=cookie and add
entry in the "Allow" section (as on the screenshot):
If the steps above are not sufficient, please try using another browser. If that also yields no positive results, please take a screenshot of the "Console" tab in your browser's Dev Tools (hit F12 when attempting to connect).
In the link below you can find a brief how-to on Dev Tools' usage: