Hi All,
Sorry for the delay reply, we have tracked this incident.
This issue was posted to the Service Health Dashboard (SHD) as incident PL215114 Start time: Friday, May 29, 2020, at 6:40 PM UTC
User Impact: Guest users may be unable to access the Planner app via Microsoft Teams.
Scope of impact: Any guest user attempting to access the Planner app via Microsoft Teams may be impacted.
Current status: We’ve received a report that impact associated with PL215114 persists, we're continuing to investigate the data gathered from our internal reproduction of the issue to identify the root cause and create a strategy to remediate impact.
Root cause: A recent change to add extra functionality to web authentication functionality resulted in impact.
Next update by: Thursday, June 4, 2020, at 5:00 PM UTC
Tenant administrators can view current information and updates on SHD at the link here
https://portal.office.com/AdminPortal/Home#/servicehealth
Yours Sincerely,
George