Oracle APEX. Problem: ERR-1016 Application "nnn" Page "xxx" not found (requested language="en

During the login process, this bug will prevent the proper authentication, but it will be gone after a second try. Nevertheless, don't let it there or the users will not be very happy.

Solution:
Application Builder > Your Application > Page 101 > Processes (Page Processing) > Login

wwv_flow_custom_auth_std.login(
   P_UNAME => :P101_USERNAME,
   P_PASSWORD => :P101_PASSWORD,
   P_SESSION_ID => v('APP_SESSION'),
   P_FLOW_PAGE => :APP_ID||':1'
   );

Just wipe out the 'about session' row! The result will be:

wwv_flow_custom_auth_std.login(
   P_UNAME => :P101_USERNAME,
   P_PASSWORD => :P101_PASSWORD,
   P_FLOW_PAGE => :APP_ID||':1'
   );

and you'll be glad to note the issue has dissapeared!