Problem
The error message detail would be different, depending on the cause of the error.
Example
Error detail:
Error when attempting to serialize the custom field XXX because Exalate does not support its type....
Cause
Outgoing sync includes the code, which tries to send all of the custom fields with their values.
replica.customFields = issue.customFields
Solution
You need to specify the custom field name when assigning the custom field to a replica variable.
Check how to synchronize custom fields for more details.