Unable to Back up On-Demand Files (code 1634)
A backup plan failed with the following error message: Some On-Demand files cannot be backed up. These files are stored only online due to File On-Demand option enabled for them
This error only affects file backup on OneDrive folder with Files On-Demand option enabled. When the Files On-Demand is enabled for your OneDrive folder, it saves disk space by not copying the file on the computer, instead, new files created online or on another device appear as online-only files in File Explorer.
Disable the File On-Demand option. Note that this will increase the disk space usage on the affected client computer. Then start the backup plan again.
Exclude affected On-Demand files from the backup dataset.