Check if you also have Axure RP7 installed. To me, it looks like Axure RP7 is launching and reporting this error (that is the logo used for Axure RP7). You might need to either uninstall RP7 or go into your OS settings to make Axure RP8 the default application associated with .rp and .rpprj files.
If this doesn’t fix it, send an email to support@axure.com and they should be able to help.