using a photos library in a nas

"there are any compatibility issues with the NAS format? "

The incompatibilities are with the file system format and network access. Photos for Mac is strictly a single user program and the internal SQLite databases in the Photos Library are meant to be used only by a single user on a local volume, not remotely. If a library is on a network volume, a user could try to access the library from several different computers at the same time. This can create problems with the file ownership. And even with only a single user accessing the Photos Library remotely, you would risk inconsistencies or risk dataloss because of possible race conditions. The network transactions will need a special synchronisation to make them safe for a network database. You have been very lucky, that the library has not been damaged on the NAS previously.

The only remote access to a Photos Library offered by Apple is iCloud Photos. We are working with a mirrored local version library on our Mac, all user interactions are handled locally, and iCloud Photos is updating the remote library in iCloud.

Hi

Nas is not a compatible storage location for a photos library.

Only directly connected exernal drives are suitable:

It must be formatted APFS or MacOS extended (journaled) (preferred for spinning drives) and not case sensitive.

It Must not be (or have been) used for time machine. (A time machine drive can be used if re-formatted)

It must be able to have the 'ignore ownership" checkbox ticked.

It should not be on a memory stick, SD or similar.

It must also not be a network drive (a direct connection is required - eg USB or Thunderbolt). File sharing services such as iCloud Drive, Drop box, Onedrive etc are not compatible with Photos libraries.

See

https://support.apple.com/en-gb/HT201517