I've just had this same error on setup of a first time key. It turned out that Plesk does not like spaces in the label at the end of the key.
i.e. the key ended in "New Key for MySite" and I had to change it to "NewKeyforMySite" - once I did that, the key manger page worked,
For anyone else looking for the mysterious icon, it's top right and used to expand/collapse the right hand panel.
There are screenshots in this thread: Resolved - How to work with SSH Keys Manager