When I connect my mixer with Ethernet and press Store to Join my network, why do I get an "Invalid SSID" error?
SSID is the technical term for the name of a WiFi network, and it is what uniquely identifies your WiFi network, not your router's LAN. It has no bearing or application whatsoever when using an Ethernet connection. See here:
NOTE: It's advisable to boot the mixer with only the network connection you plan to use (wired or wifi) and disconnect the other, so there's no confusion or chance of duplicate network connections.
When using a hardwired connection to the router, there's no joining of anything necessary. If you're connected to your router via ethernet, there is no SSID (wireless network name) involved. Once connected to the router with Ethernet (and with the WiFi dongle removed), the Network page of the system menu should look something like this (depending on your firmware version):
Connected To: Ethernet (because you're connected via Ethernet)
IP Address: a.b.c.d (the IP address that the mixer gets from the router)
WiFi Networks: None (as long as the wifi dongle is removed)
You do not have to press Store to Join in order to connect to the router via ethernet. If the ethernet cable is connected when the mixer boots, the mixer detects this and automatically gets an IP address from the router. Once you see an IP address listed, the mixer is on the network. You only have to press Store to Join in order to connect to a wireless network.
The reason you get the "Invalid SSID" error when using Ethernet and pressing Store to Join is because when the WiFi dongle is not present (it should be removed if you're using Ethernet), there is no way for the mixer to detect any WiFi networks and it will say WiFi Networks: None. There is no SSID to join, so pressing Store to Join results in it the "Invalid SSID" error because you're trying to join a blank or non-existent SSID.