This is related to a corrupted known_hosts file, generally a new line character.
But ssh user@hostname doesn't help you figure out which line is the issue.
Try using ssh-key -f "~/.ssh/known_hosts" -R <hostname>
root@jackson:~/.ssh# ssh-keygen -f "/root/.ssh/known_hosts" -R vertex buffer_get_ret: trying to get more bytes 7 than in buffer 2 buffer_get_string_ret: buffer_get failed key_from_blob: can't read key type key_read: key_from_blob AAAAB3Nz failed line 10 invalid key: |1|frpMH8CpsRplzcHhR1P7R0ktEoQ=|mMYvdIry...
Once you find Offending Line:
sed -i '10d' ~/.ssh/known_hosts