NAS Asustor AS3202T: Login fails after upgrade to 12.0.4

Asustor NAS AS3202T
Nextcloud version (eg, 12.0.2): 12.0.4
Operating system and version (eg, Ubuntu 17.04): ADM 3.0.4.RCQ1

The issue you are facing:

I had installed Nextcloud 12.0.0 from the Asustor App Central (last year).
Yesterday, after upgrade to 12.0.4 using first the built-in upgrader and finishing with command ./occ upgrade,
everything seems to be working. But I cannot login through the browser interface (as admin).
I cannot find anything relevant in the log files.
I have run command ./occ maintenance:repair