An issue does not get synchronized and appears in the troubleshooting section with the category Custom Field Mapping Error and a problem description similar to "Custom Field Mapping '...' failed: ...".
The reason for this error is that one of your custom field mappings raised an exception while trying to map the field data.
Please reach out to the developer who has implemented the custom field mapping. If you are the responsible developer reading this now, please take a look into the Jira log files and investigate the stack trace of the error. If you need further help, please reach out to email@example.com or raise a ticket here.