I noticed the same problem occurring when a plugin or code modifies the ViewTemplates.
Since v5.2.1 the cascades mechanism has changed the way ViewTemplates work, so old code can break post 5.2.1 tiddlywikis.
I came up with a workaround : create a new tiddler and paste this code in it:
{{$:/Import||$:/core/ui/ViewTemplate/body/import}}
It should allow you to import your files.