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I just started having similar problem. I have different SSID for each channel, 2.4 and 5.0. Both have same password. When I switch to 2.4 I get the incorrect password and enter the password and it works. When I switch to 5.0 it does so no problem. I’ve tried several recommendations, including reboot modem , reset network on phone, forget network and reenact. Restarted phones as well. Nothing helps. I have IPhone X and Xfinity Xfi modem. This just started. Oh, happened on wife’s phone only once.