Hi @Kurt Stichelmans ,
Can you try creating a new Sharing policy as below and assign that to the mailbox and test it.
New-SharingPolicy -Name "ExternalSharing" -Domains 'Anonymous: CalendarSharingFreeBusySimple' -Enabled $true
Set-Mailbox -Identity <user name> -SharingPolicy "ExternalSharing"
Please try to share calendar using OWA and check if that works.
Sometimes this error in Outlook could be Autocache. Try deleting the address in "To" field while sharing calendar and re-type the full email address of the external recipient.
If the above suggestion helps, please click on "Accept Answer" and upvote it