When adding an external file server to the IMP, the External File Server Public Key has a limited number of characters and cannot be fully entered into the CentOS Public Key. The file server is CentOS8.
Also, note the documentation for IM&P version 11.5(1).X and 12.5(1).X:
External File Server Requirements
Follow these guidelines when setting up your external file server:
Subject to file server capacity, each IM and Presence Service node requires its own unique Cisco XCP File Transfer Manager file server directory, however, nodes can share the same physical file server installation.
The file server must support an ext4 file system, SSHv2, and SSH tools.
The file server must support OpenSSH version between 4.9 and 6.x.