looks like the password for the user still has the „old“ format somewhere (there has been a change in the hashing mechanism quite a while ago).
First you can go to the ServerManager, „edit users“ and look for the user in question in the user list - if it is highlighted yellow there, that means that the user itself has the old password hash format. If not, the technical user information in the remote project configuration might still be in the old format.
I‘m not sure if that is really the root cause, anyway a simple approach to try would be to resave the user‘s password so that it will be hashed using the new format.
Background informationen: FirstSpirit never saves user passwords but only their hashed values.