Such FH maps as Ficho Tunnel, Fluorite Plains, Lonely Cave, Acsension Island, Sky's Rim, Atlantis, South Pole and the Last Cave don't exist in FH+. So, if you left your character on these maps in FH and then went in FH+, it will send you to a pretty scary black hole with no title. You can leave your character in Temple of Dreams, Bonfire Island or Cape of Distant Worlds on FH so that if you go to FH+, you will be able to play normally.
And try to place all regular FH maps in the FH+ folder. You will be able to go to FH+ maps from FH ones if it's needed, but you won't get back to FH maps there;)