Demystifying Hybrid Free/Busy: what are the moving parts?

Yaseenhh , I work in Exchange Online Support team where I handle cloud free/busy and hybrid free/busy. The scenario you are describing seems pure Exchange On-Premises (cross-site Free/Busy). However, the Free/Busy call should be the same (GetUserAvailability). You need to retrieve that call and the error response. For the error message, you can use F/B test on Remote Connectivity Analyzer https://testconnectivity.microsoft.com/tests/FreeBusy/input  (source user = user from site1 , target user = user from site 2) or Fiddler tool while you reproduce the issue in Outlook or OWA client in Scheduling Assistant as explained in the second part of the blog: https://techcommunity.microsoft.com/t5/exchange-team-blog/demystifying-hybrid-free-busy-finding-errors-and-troubleshooting/ba-p/607727.