Which version of Microsoft Tape Format is supported by which MS SQL Server
The problem was indeed a corrupted backup, as Kin correctly deduced. Most likely the corruption was introduced during an ftp transfer.

So, when you get the error "The media family on device 'C:\myfile.bak' was created using Microsoft Tape Format version 1.8. SQL Server supports version 1.0." or the error "Specified cast is not valid. (SqlManagerUI)" when using the SQL Management studio Gui to restore a backup, you most likely have a corrupted backup file.

In my case, zipping the file before ftp transfer solved the problem.

You would think that backup files always contain a checksum, and if the file was later modified that sql server would report this incorrect checksum, but you would be wrong.