Help! In one TW of mine the TagManager is broken. When I call it (menu > More > Tags > tag manager) I see a tiddler with the right name, but it is empty. When I start the editor however, I can see the right content in the view window of the editor.
I copied the TagManager tiddler from another TW, but this did not help so I deleted the copy.
I am aware that the TagManager is a system tiddler, but I do not know if that makes a difference in f.i. the ability to import it from another TW by dragging a link (f.i. the link at tiddlywiki.com that you gave, or from a TW where it still works OK).
A better alternative to getting a copy of it, is to just delete it and the shadow tiddler will be restored.
Also go to advanced search, Filter and select from the dropdown select “Overridden Shadow tiddlers” these are core tiddlers that have being modified, if you edit and preview them you can see the “differences from the shadow” and possibly locate a change causing the problem.
Other possible causes,
If you have being configuring CSS perhaps you have hidden the content somehow?
If you have being customising the view template body cascade that treats TagManager differently?
Also remember there is also an Unfold all button on the Page Controls on the sidebar. If you tend not to fold things and see something similar try that. Its a Pity there is not another indicator that is clear when folded.