When updating an item in a custom list I want to copy the updates to a another custom list with exactly the same columns.

I have created a simple Nintex list workflow running on modified items. As the target list is in another site collection I'm using the  "Office 365 update items" action.

This goes fine with text columns, note columns, and single value option columns.

But I also want to copy updates between user columns, multi value option columns and multi value lookup columns.

I keep running into two scenarios - either an empty value is copied into the target column or the workflow fails to run with this message:

>InputMetaDataXml] : The 'Fields to update' is invalid. Input string was not in a correct format

I have tried various combinations of the available source and target data formats but haven't found the right combination yet :-(

I would expect that no conversion should be necessary as the target and source columns are of the exact same type.

Please point me to the authoritative guide to the "Office 365 update items" action.