Accessing office.com could loop continuously to the CASB Gateway SSO helper plugin page.
Symantec has a fix with CloudSOC 2.109 released on March 14, 2019 for the US and March 11, 2019 for the EU.
In chrome, check the box for bypassing service workers. F12 | Applications | Service Workers | Bypass for network.
Symantec recommends using the Casb Reach Agent, or proxy chaining such as ProxySG or WSS integration to avoid browser issues or limitations with the SSO helper.