An impediment to resolution has been lack of user detail on how this happens, after they solve their issue.
Problem Backing up in w7 is only waiting two days, so might still have some hope of getting further details.
FileNotFoundException (temp file not found during backup) is at 17 days, and there are lots of older cases.
I would suggest a retry-level log, and in order to make it easier to gather, I will suggest doing it to a file, like:
–log-file and log-file-log-level in Advanced options. There’s increasing evidence (see earlier cases) that an earlier problem causes this one. Note that there are several paths to “Could not find file”, so if you can find some existing error with a multi-line stack trace, please post. Possibly About → Show log → Stored has your previous ones. Click on them to see the detailed stack trace, then run a log to see where it had failed.
This indicates that the previous backup was interrupted, which fits in well with its failing due to some error.
This probably happened near start of the run. Duplicati tries to upload a synthetic backup version showing backup progress before interruption (based on backup files before that, plus progress before interruption).
Fix synthetic filelist not being uploaded #4114 fixed bug causing the error, but fix is not yet in Beta release.
EDIT:
Aside from being a more permanent collector, this is the other reason a file will work better than “live log”.
You might also have a look at your past job logs to look for non-zero RetryAttempts stat in the Full log.
What Storage Type is the Destination? My Google Drive seems especially likely to do random Forbidden