Quickly said, the payed version offers the same features + all the furniture and textures libraries available on sweethome3d.com. So sorry, don't hope that the payed version will offer some bug fixes that wouldn't be available in the GNU GPL version (or very temporarily).
You could try older versions like the version 4.6 which used an older Java version. If it works, then replace the *.jar files found in [font=courier new]lib[/font] subfolder of the installation folder with the ones of the latest version (therefore make a copy them before uninstalling the version of Sweet Home 3D you have).