The error message detail would be different, depending on the cause of the error. 


Error Details Message
Error when attempting to serialize the custom field XXX because Exalate does not support its type....


Outgoing sync(data filter) includes the code, which tries to send all of the custom fields with their values.

replica.customFields = issue.customFields 


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.

See also

Custom fields synchronization