If a user receives a host error: "Either the host is not available, or the file is not available on that host" after the transition of a FileMaker server, it is because the client has cached the DNS name to the old IP address. The user can verify the version they are using by going to File Maker Pro Advanced > About while the client is open. These are the steps to remove the old host on the client and adding back in the new one:

v16

• File > Open Remote
• Select the server from the list, and click the currently selected "star" icon to remove it as a favorite.
• Go back to File > Open Remote 
• Click the + icon and type in the DNS name of the new server in both "Host's Internet Address" and "Favorite Host's Name".
• Click save or add.
• Click on the host and click the star icon back to "favorite" it.

v17

• File > Hosts > Show Hosts
• Right click on the server and "remove favorite"
• Click the + icon and type in the DNS name of the new server in both "Host's Internet Address" and "Favorite Host's Name".
• Click save.

References:

v16: https://support.claris.com/s/article/Saving-edi...

v17: https://fmhelp.filemaker.com/help/17/fmp/en/i...