I have got Mi 8 running lineage OS 17.1, got transport error too. This is how I resolve the problem:
- Make sure your phone screen is on and unlocked.
- You can do this either in root or non root mode.
- run "adb shell" to get a cli prompt, or a terminal or ssh to it if you have Termux set up.
- run "bmgr run" to begin the process.
- run "bmgr list transports" to identify your current transport. In my case, it is "com.google.android.gms/.backup.BackupTransportService"
For example: -
dipper:/ # bmgr list transports
com.android.localtransport/.LocalTransport
com.stevesoltys.seedvault.transport.ConfigurableBackupTransport
com.google.android.gms/.backup.migrate.service.D2dTransport
* com.google.android.gms/.backup.BackupTransportService - If the transport is not "com.google.android.gms/.backup.BackupTransportService", you can set it by running "bmgr transport -c com.google.android.gms/.backup.BackupTransportService" and run "bmgr list transports" again to confirm. The selected transport is with * prefix.
- run "bmgr init com.google.android.gms/.backup.BackupTransportService" to re-initialise the transport.
- Run "bmgr backupnow --all", and good luck.