Here is a couple of possible causes:
1. Wireless Sync on the device calendar is set to "No" or the option isn't present. To check go open "Calendar > Press menu (Left of the trackball) > Select Options > Select Desktop > Ensure Wireless Sync is set to Yes".
2. The incorrect cdo.dll file is registered on the BES. Ideally there should be only one instance of cdo.dll present on BES machine under \Program Files\ExchangeMapi\cdo.dll. It is installed as a part of MAPI/CDO (please see step 1 of zcb install doc). In case if determined that multiple cdo.dll's are present, please register the one mentioned above.
Please also find relevant articles below:
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