MacOS – NAS – “there was a problem connecting to the server” while on other networks

AFP mounts are tied to both a path and an authentication.

An exported volume might have multiple paths. For example, an OS X server might export its root, the /Users/ folder, and the /Groups/ folder. Each of these can be mounted separately. In your case, your NAS may be exporting its volume under multiple paths. It might even be exporting the same path multiple times. Check the NAS' settings.

Each AFP mount is tied to a particular authentication (username/password). You can mount the same volume/path multiple times using different authentications. Its possible that the credentials you used in those two different places are subtly different. Depending on the NAS, it might even come down to whether your long username or short username are used. Your NAS might not even require a username (only a password) and might ignore whatever username is given. OS X has no way of knowing that and so two different usernames (long versus short) might produce two different mounts even though they lead to the exact same volume.

You can do this by making a symlink (using Terminal, not an alias in the Fnder) to the directory on the NAS as you describe, but I'd caution against it unless your laptop is alway connected to your network. You could (and most likely will) get some unpredictable and possibly data corrupting behaviour if the Mail app tries to access data only to find the network volume isn't mounted or accessible.