Thank you for contacting us.
The bug you are describing is in our known issues
for Sitefinity 5.0 release. The problem is that when you create a new taxonomy it has a unique GUID identifier and if you try to create the same taxonomy in another project even it has the same name and properties still it has got a different GUID. And there is no easy way to match those GUIDs when exporting and importing modules. What I am suggesting is before exporting the module edit the fields, delete the custom classification field and then save. After that export the module and import it where you need it. Then edit the fields again and add your custom classification field.
I apologize for any inconvenience this is causing.
the Telerik team
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