Hello,
Are you trying to run dbman manually on the DB Server? That is no longer supported for some time now, and there may be issues, also because of the addition of the security files I mentioned. If they don't exist on the DB server you could run into errors like you are seeing here.
To run dbman properly on the DB server, please install DMA by running installslave.bat (in E0705 installation files) and following the wizard, connecting it to the IP of the iMC system. Doing so will ensure dbman has the proper configuration files to work. Remember to restart iMC afterwards - you should see the dbman.exe and tftpserver.exe processes running on the IP of the DB server on DMA 'process' tab.
Then try the backup again. Note that if there's still an issue, in Domain environments you may also need to grant 'Full Control' permission to local 'Users' group for the iMC\dbman\bak folder on the servers - it's where the DB backups are temporarily stored before being transferred and moved to the destination folder.
Dbman backs up not only the databases, but also important configuration and security files on the filesystem of the iMC server(s). Hence backup via other solutions like SQL Server backup will not work properly, you will definitely run into issues during the migration if you tried that.