I have several AUP3 files saved on an external NAS mounted via SMB 3
Issue 1: trying to load the AUP3 file in Audacity 3.3.3 Mac, either by double clicking via Finder or via File > Open, results in a “Failed to open database file” error
Issue 2: save [new] AUP3 to NAS (e.g. via Save Project / As) results in file appearing briefly in the OS X Finder window, then the file will disappear after 2-3 seconds
Audacity can read/write AIFF and other files from the NAS just fine, the problem seems isolated to AUP3 files on NAS.
Workarounds:
To read AUP3 file from NAS: copy the AUP3 file from the NAS to a local directory
To save audio file to NAS: export the audio as AIFF or other format to the NAS
Notes:
AUP3 files were created by Audacity 3.3.3 Windows 11, which are able to read / write to the NAS fine. This was tested using various computers including a Windows 11 VM within the same Mac
Good plan. Audacity uses SQLITE3 as its database engine to drive access to its unitary project .aup3 file. There are limitations when using this engine for databases residing or distributed over a network as outlined here: SQLite Over a Network, Caveats and Considerations
Manual copying of the project file between local and remote storage is recommended.
There should not be an issue with other types of files.
Thank you. This is good to know. It would be nice to have as feature request a more NAS/network tolerant format planned for AUP4 as these are becoming more common and tablets are also becoming much more powerful.
I’m revisiting this thread. Although I understand the limitations of SQLite, the fact is:
AUP3 on NAS works fine on Windows, even when Windows is running on a VM within the same Mac
There are many issues with Audacity over OS X, such as the crash on open and pointer lag issues. Compared to 3.3.3 on Windows, the OS X version is far less stable. For this reason the AUP3 over NAS issue reported here should be investigated
If SQLite is truly so bad over network drives, Audacity should not be using such fragile technology for the AUP3 format
Networked drives, OneDrive, iCloud, NAS, etc. are a fact of life nowadays. Can’t really brush this off to “Audacity doesn’t work with networked drives”
Not saying I will edit AUP3 over NAS, but this issue should be investigated.
Audacity uses SQLITE3 as its database engine to drive access to its unitary project .aup3 file. There are limitations when using this engine for databases residing or distributed over a network as outlined here: 
as these are becoming more common and tablets are also becoming much more powerful.