Activesyn not working as intended

Activesyn not working as intended

hello,

i hope someone can help me resolve my problem with syncing.

What is working:
contact sync:
      Outlook (Desktop App) <-> Zoho
      No issuses with contacts
Calendar:
      Outlook (Desktop App) -> Zoho - NEW EVENT -> not syncing
      Outlook (Desktop App) -> Zoho - not syncing modifications, event created in zoho app/webmail
      zoho -> Outlook (Desktop App) - syncing everything as intended

Outlook Desktop App, latest version -> Exchange ActiveSync -> msync.zoho.eu

Thank you,
Yves