My backup software quit working yesterday. Can you help me fix it?

I have used Dantz Retrospect Express 6.5 backup software for several years. I cleaned some files from my hard drive yesterday and now Retrospect won't start. The error message says "Configuration file 'config65.dat' is not writable." I can open the file and it seems to be okay. I uninstalled and reinstalled Retrospect, but nothing changed. Is this file unique to Retrospect, or is it also used with other programs? What would make the file "not writable"?