I have a 2860+ running firmware 3.8.9.1_BT
Like several other reports there have been many instances of my iPhone, iPads advising of 'wrong password' and other devices failing to connect or 'dropping off' the Wifi.

This morning, a wifi scanning app showed me that there was no RF with the expected SSID, I was able to look at the router's wireless settings and discovered that 2.4GHz wifi was turn off i.e. in 'Wireless LAN(2.4GHz)>>General Setup' the 'Enable Wireless LAN' BOX was unchecked !
Clearly, overnight some internal gremlin had turned it off ... as I hadn't.

Hello

Can you post your current settings? All tabs

Spellbinder : can you be more specific ... do you mean all WiFI settings, or all the router settings ?

Also, would you like 'screen capture' .png files or a text file ?

Hello

All wireless settings menus ;)

You can post the screenshots (remove/hide any sensitive information) on this website https://postimages.org/ and paste the links here

The bug you're describing sounds very similar to this fix listed in the Vigor 2862's 3.8.9.2 firmware release notes:
Wireless clients connecting to 2.4GHz wireless SSID and disconnecting could not connect to 5GHz wireless SSID, receiving an invalid security key error

Since most Apple devices are dual band capable, it's more likely that you'd see the problem with iPads / iPhones. Update to 3.8.9.2 and see if that fixes it.

In regards to the wireless unexpectedly being off, check whether the wireless LAN button on the front of the router was possibly pressed - pressing it for 1 second will toggle through the 2.4GHz & 5GHz interfaces on and off in each combination (on/on, on/off, off/on, off/off)

Beyond that, check the syslog output, it should be giving log information when it's doing Dynamic Frequency Selection checks, during which time the router won't be broadcasting on the 5GHz band.