Hive account@antisocialist has a little more experience with this than me and correctly points out that sometimes when you copy a private key from a text file it will add an empty space " " at the end of the key that will automatically make the key invalid.

Luckily I just tested it and this doesn't seem to be an issue on the website you're currently using... but it could be the reason why other attempts have failed on sites like Hive.Blog