I have just come across this bug. It seems to be intermittent.
However, this error will leave "Unfinished large files" in the B2 bucket. This can cause your B2 storage to be significantly above what you might expect.
The "Unfinished large files" can be seen by logging into your account and going to Buckets. (You can also check the cpbackup_transporter.log for the api error.)
In your account, the bucket will show "Unfinished Large Files" which you can click on to see the list of files. The "Current Size" as shown in Buckets will be larger than the size shown if you go to Browse Files and then click into the bucket. The unfinished files can be deleted by browsing into the bucket and the relevant folder.
Note that since the cPanel Backup app tries a number of times to upload files, there can be "Unfinished large files" in the bucket even if you don't receive a notification of a backup failure.
Anyone who is currently using the cPanel backup app to upload to Backblaze B2 should check their bucket for "Unfinished large files". Depending upon the number of failures and the size of the files, the total size of these can be very significant.