Demystifying Hybrid Free/Busy: Finding errors and troubleshooting

chris280 , I assume it is from EXO to Exchange onpremises? it fails at ews request and usually network related issues. You need to check httpproxy logs / iis logs to see if the request reaches Exchange when you repro the issue or it is stuck at some network device in front. If you are in Modern Hybrid Topology and your Cloud IntraOrganizationConnector or OrganizationRelationship's TargetSharingEpr is pointing to the Hybrid Agent, you need to see if the agent is active /still installed and your firewall/ proxy is allowing outbound traffic to EXO. You can check the troubleshooting suggestions from Scenario 1 from my other blog on Hybrid Migration Endpoints (EWS), these are applicable also here if f/b direction is cloud -> onpremises: https://techcommunity.microsoft.com/t5/exchange-team-blog/troubleshooting-hybrid-migration-endpoints-in-classic-and-modern/ba-p/953006