Please install the newest Office administrator templates. Could you please share us which GPOs did you set for the default saving location?
On my tested in my environment, I set the Default File Location via group policy below, they work fine.
- Word: Microsoft Word 2016/Word Options/Advanced/File Locations --- Default File Location
- Excel: Microsoft Excel 2016/Excel Options/Save --- Default file location
Besides, the location seems to be a network location not a mapped drive, I suggest you check other mapped drives and use a UNC path for a check.