Aye, its probably because of how many users are on right now. There is about 900 people on, so maybe if you wait a little bit, or you keep retrying, you may be able to get in. If the character you are trying to get into, is in fluorite, this is probably the reason. fluorite is the most popular place in FH, so it slows down the servers by alot. If it is not the number of users, then id suggest redownloading FH. Sorry >.<
Solution: Just go on another character that is not in fluorite. Wait for the number of users to go down before going on your character. Maybe come back later and try again. Keep trying. Or if all else fails, try redownloading FH.
Look on the right hand side of this site to look how many users are on.
Hope this helped <33 ^^
~Rawy