Looking at it.  The tab settings coming out of v17 look a little odd, in that it's not a format I would have expected v17 to write out, but it's a format we're supposed to be able to handle, so there's a bug here somewhere.  
If you want to get it running for now, two options:
1) Remove the 18.0.0 directory from your config directory.  (ie: ~/.slickedit/18.0.0 under *nix, My Documents\My Slickedit Config\18.0.0 for windows, HOME/Library/Application Support/Slickedit/18.0.0 on Mac).  Fire up v17.  Change the C++ tab settings to something like +4.  Exit v17.  If it's set up to prompt you to save the configuration, be sure to tell it to save.  Start up v18 again.  If the tab setting was the problem, then it should come up with your settings imported.
2) Start it with a clean config.  Start v18 with the command line options "+new -sc SOME_DIRECTORY" where SOME_DIRECTORY will be the location of the new configuration directory.  Should start up.  At this point, you can start up v17, export your configuration via the options dialog, and then import that into v18.  I suspect this will work even though the initial import didn't, but I can't be sure yet.