I have created a site via SharePoint which has a planner built it. When I access my site i get an error with the planner error "Something went wrong. Can't show this plan. Please try again later." however i can access the planner if i do not go through the site.
Therefore i believe this rules at a possible network issue as I can access the planner, just can't access the planner through a site.
This issue happens on all the PC's/laptops in my office. either on the Teams application or via office.com. I have also added the below websites as a trusted site.
UPDATE- We have tested a device on mobile tethering and the planner works. however the issue is still not resolved on the local network. We have also checked firewall rules and no outbound traffic is blocked.