RT-AC68U disconnects phone but not laptop

Hi,

I have a Sony Vaio laptop and a Sony Xperia Z1C smartphone.
Since today I have the following issue with my RT-AC68U
- connecting the laptop to the 2.4 GHz is working OK
- connecting the Xperia to the 2.4 GHz is just for a second, than it drops
- connecting the Xperia to the 5 GHz is working OK

The Xperia is not remembering the WPA2 code for the 2.4 GHz, while it does for the 5.

RT-AC68U has firmware 378.7498

I tried the AC68U in both router and AP mode: same story.

It used to work...

RT-AC68U disconnects phone but not laptop