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Figured it out:
Obviously a symbol is only created in your main.js if you position one instance somewhere on or off stage, it does not suffice to have it in your library. The createjs exporter will ignore it. Is this a bug or intended behaviour? I found also that if you are converting a bunch of AS3 linked symbols createjs will concatenate the linkage ID of said symbols to sth like srcuiButton (if you linked it in AS src.ui.Button). Better get rid of this linkages before importing to html5/canvas.
It can be just in the library. It is confusing and not obvious, but although the linkage is grayed out in the info box, you can double-click on the linkage part of the library symbol, and type in the linkage name that you want.