Forum Discussion
On prem users unable to load 365 user's calendars
BSHCOWFor hybrid collaboration, I would always recommend that the two mailboxes be in the same Exchange organisation (either on-prem or migrated together to EXO). Depending on how you've provided permissions this scenario may not be supported (details here). Even if it is supported, I've found it's a much better user experience if you just stick to a blanket rule that says if you want to access another mailbox it needs to be located in the same place as yours!
Unless I am not reading that link right, it talks about send as and delegation. They are just trying to view a user's calendar in Outlook. This is simply a user given viewer permissions on a calendar trying to view it.
Again it depends what the users are trying to do. If they are loading a calendar in Outlook, then that's a bit different then viewing free/busy information in a meeting invite. Specific calendar permissions used when opening a calendar in Outlook are considered folder permissions which comes under the "not supported" section. If it's free/busy you have an issue with, then you can troubleshoot that here https://learn.microsoft.com/en-us/exchange/troubleshoot/calendars/troubleshoot-freebusy-issues-in-exchange-hybrid