You issue is that your user does not have rights to create a record in the System.FileDocument entity. In version 5 of the modeler, you could set permissions for File objects in the Project > Security > File Access tab, although this was a legacy way of doing this.
In version 6 of the modeler, this has been removed. To set permissions on a fileDocument, you need to create a new entity in your own module (not the System module) and set this to inherit from the System.FileDocument entity. Then you can set proper access control permissions on your entity to control who can upload or delete file documents. In your export microflows, you then create an instance of your entity, instead of the System.FileDocument entity.
1) migrate to 6.3.1 2) where do you get the error - running of the modeler - running sandbox - running cloudslot 3) how is security set-up? Depending on WHEN/WHERE you get the error there are several possible issues/solutions 4) do you get the same error on different actions?