Learned from further testing: the category itself survives the transfer to Commons Beta intact. This error only shows up in the metadata *preview* (when GWToolset parses the first three records and shows the results). Hence this is a more minor error than my first message suggested.

@MartinPoulter Could you attach a test XML file that triggered this behaviour?

Note that it's not the XML itself that triggers the behaviour. In the GWToolset form, after I've chosen the file, one of the form entries asks for categories that will be applied to all the uploaded files. When I enter the category "Files uploaded by the Bodleian Libraries' Wikimedian In Residence" (without quotes) the next step is the example three metadata records, and it's at this stage that I get the error.

In T125202#2001484, @MartinPoulter wrote:

Note that it's not the XML itself that triggers the behaviour. In the GWToolset form, after I've chosen the file, one of the form entries asks for categories that will be applied to all the uploaded files. When I enter the category "Files uploaded by the Bodleian Libraries' Wikimedian In Residence" (without quotes) the next step is the example three metadata records, and it's at this stage that I get the error.

Right, my mistake. Thanks for clarifying :)