Did backup or restore work before (if so, how long, and what changed), or are you still getting started?
“One or more errors occurred” is (I assume) the B2 upload problem. With neither upload nor download working well, I suspect you have network issues even though some other downloader (what) does OK.
Duplicati.CommandLine.BackendTool.exe would be the Duplicati pure downloader (or uploader) which could do a test without the extra encryption and compression loads that actual backup or restore have.
Have you looked at Task Manager to see if there’s much Disk or CPU loading? Keep your core count in mind, so for example a four core system might be limited to 25% loading at Performance → CPU stats.
In addition to testing Duplicati standalone download and upload speed, you could try the Speed Test at DSLReports, with Settings set to single stream to more realistically compare to Duplicati’s network use:
Are you WiFi connected? Sometimes a physical connection works better. Any other computers to test?
Duplicati.CommandLine.BackendTester.exe can do an automated test involving upload and download.
This (like BackendTool) requires a URL which is similar to what Export As Command-line show to you.
Unless there’s a CPU or Disk limitation visible in Task Manager, this might turn into network debugging.
What’s your level of system performance or network troubleshooting skill, so I can try to adjust for that?