We are required to move away from Office 365 and external drives (OneDrive, Dropbox) to internal NAS for security reasons and evaluating Office 2019 with internal NAS.
Things were going well until we found that earlier releases of Office (2010) could not use the 2019 Excel files when saved to NAS. The Title Bar and 5 lines of the the spread sheet are moved under the Menu and Task Bar targets and can't be moved into the workspace. Several damaged engineering worksheets with heavy VB were found blank, but could be opened by using "New Window" page, but then didn't operate.
Further testing shows coping 2019 Excel files onto USB drives and Internal NAS (Synology family) result in the problem. Then testing on Dropbox and Google Drive show the same problem. The only way to fix it is transferring through OneDrive which is excluded by the insurance underwriter.
Our thinking this is a fatal flaw in 365 and 2019 and to drop back to earlier office versions.