Could be many issues; need to narrow down root cause. Start with these
articles and verify some settings are correct.
Exchange: ESM Missing Public Folder Tree
Use this article below to ensure that your sitefolderserver is
pointing to a valid Exchange server.
Schedule+ Free/Busy system folder is missing
James Chong (MVP)
MCITP | EMA; MCSE | M+, S+,
Security+, Project+, ITIL