The following error is thrown for a custom field when running a synchronization:
" The mapping rule (CCC_proxyAddresses <-> proxyAddresses) was unable to execute synchronization between system objects successfully!
 Property (CCC_proxyAddresses@ADSAccount) could not set the value (System.String) from system object (objectname)!
 Schema property (CCC_proxyAddresses@ADSAccount) only accepts data of type (System.String). The value to write (System.String) is however type (System.String)."
The "Data type" of the custom column should be set to Text, not String.
Additionally, the "Multi-value" checkbox must be selected for the custom field. This option can be viewed in the Synchronization mapping but it can only be edited in Designer.