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You can find your preference files in the TFDIR_PREFS folder.
The structural preferences are stored in a binary resource file, stfprefs.rsc. When the software is not running, you can try backing up this file by renaming it to something like stfprefs_rsc.001. If you do this and restart the product, you will start with the default "out-of-the-box" preferences. See if that makes a difference.
If that doesn't work, there are a couple of other preference files such as structural.upf (or whatever user name you're using) and tfprefs.rsc in the same folder and you can try the same thing with those. Write back and let us know your results.