There is an issue with Unity where it loses the link to the font file in the demos. This can be fixed by doing the following.
1. Open up the affected sprite collection. (TK2DROOT/tk2dUI_demo/Assets/SpriteCollection/UIDemoSpriteCollection or TK2DROOT/tk2dUI_demo/Assets/Dark/UIDemoDark).
2. Find the entries under the heading "Fonts". There should be one or more with the name "Empty". These are the ones which have lost their links.
3. Fix up the links as shown in the attached image. i.e. Click on the texture to highlight the location & name, find the corresponding text file and drag it into the BM Font slot.
4. Repeat for the other "Empty" fonts.
5. Click Commit to commit the sprite collection.
We tracked it down to exactly what Unity does to cause this, but haven't been able to reliable reproduce this to submit a bug report to Unity. If you can reliably reproduce this, please get in touch - we'd like to narrow this down and submit a bug report.