378.55 on both AC66U and AC87U and cannot login anymore

Hello,
Last week I updated the firmware of my two routers Asus RT-AC66U and RT-AC87U from stock Asus to Merlin 378.55.
Everything went well, Asus device discovery keep seeing both, but the last couple of days I am not able anymore to log in to any one of the routers.
I tried with Firefox (40.0.3), Chrome and I Explorer but same results...
Tried reboot the routers, clean the browser cache, same result..
Any idea about why and how I can solve this problem ?
Thanks