This is all about Speed Notes - not Notes. I suspect that file folders have been moved around manually but incorrectly.
John is absolutely correct, what you are seeing is speed notes that have been placed under the SoftPlan\Symbols folder rather that under the SoftPlan\Speed Notes folder. When in that location there will not be any Preview as the symbol selection dialog preview only shows 3D Symbols, which speed notes are not. This behavior has not changed from prior versions of SoftPlan.
To correct this, simply move your custom speed note libraries from the SoftPlan\Symbols folder to the SoftPlan\Speed Notes folder and then access your speed note libraries from the Notes > Speed Notes... menu.
I hope this clarifies what is happening.