My apologies in the delay in getting back to you.
There are a few things I can recommend you try and then see if the error still occurs after each one. I've listed them below.
1. Manually uninstall PlusSpec, then reinstall it again. There is a section at the bottom of our FAQ page which explains how to do this. The link to this page is below.http://plusspec.com/faqs/
2. Manually uninstall PlusSpec again and uninstall SketchUp, then reinstall both. Please note that you may need to enter your SketchUp license again. The link to download SketchUp is below.http://www.sketchup.com/download/all
3. Update Safari to the latest version. You will need to update to the latest OSX version as well if you haven't already done so.
Also, I read earlier that you installed PlusSpec on a second computer and was wondering is it still working fine on this computer?