SUMMARY
When using a remote database (such as on a NAS for multiple users) it is impossible to use Digikam when you are no longer on the same network as the remote database. You can get around this issue with a VPN, but the slow connection causes Digikam to be so slow it is very difficult or impossible to be productive, even when only working with local files. In cases where you have no internet connection, such as on a plane, it is impossible to even start Digikam.

Digikam should use a separate database for local and remote collections so that local collections can be used without a connection to the remote database. This would also greatly speed up local file operations when using a slow connection (such as a VPN) and possibly reduce the load on the network and remote database which could speed up remote database operations in some circumstances.

ADDITIONAL INFORMATION
This could be implemented in two different ways. While I would like local and remote collections to use separate databases but be accessible at the same time, I realize that this could be a very challenging and difficult feature to implement. Because of this, I suggest an alternative solution where the user can select which database to use during boot. This could allow the user to use each database/collection separately and possibly open two instances of Digikam; one for local files and one for remote files. In this case, an option to migrate a single image or selection of images from one of the databases to the other could add additional functionality to the simpler option without being over-complicated.