It's a known issue.
There's a workaround, however.

1. Launch GameSave Manager
   
2. From the Main Menu, click the 'Program Settings' option
   
3. Select 'Task Specific -> Backup Settings' from the Categories panel on the left of the screen
   
4. Tick the 'Set a default Backup Directory' option. This'll display an input field and Browse button.
   Click said Browse button and select your OneDrive sync directory

Due to this workaround, I've opted to not try and fix this issue in an effort to keep more focus on the 4.x branch development.

引用自 InsaneMatt：

It's a known issue.
There's a workaround, however.

1. Launch GameSave Manager
   
2. From the Main Menu, click the 'Program Settings' option
   
3. Select 'Task Specific -> Backup Settings' from the Categories panel on the left of the screen
   
4. Tick the 'Set a default Backup Directory' option. This'll display an input field and Browse button.
   Click said Browse button and select your OneDrive sync directory

Due to this workaround, I've opted to not try and fix this issue in an effort to keep more focus on the 4.x branch development.

Thanks for the answer. I wouldn't call it a work around, as it become a normal backup and not a cloud one (kind of). So manually setting the directory is the same.
And since it was for a schedule task.

I would agree on focusing on next version, if you also can have that fixed at the same time. It is not a huge issue at all.