Just to be sure... you have .rli files associated with RWPaint in Windows and you double-click the files in Explorer to open them and then this message appears?
The first thing I would do, is to open RWPaint (old or new) manually and then open the files from within it. If it works from there, maybe the associations needs to be refreshed. If it cannot be opened, you can grab the older and newer versions from here and replace the files. Maybe something went wrong with them. (Also, check for viruses, maybe the files got infected.)