During extensive testing of various versions to trace down my problem
http://forum.mikrotik.com/t/upgrade-hotspot-3-23-3-28-login-broken/31379/1
I found out, that upgrading from 3.27 to 3.28 with ISO looses the erases the configuration.
When upgrading from 3.23 to 3.27, using ISO as well, I was asked
“Do you want to save old configuration [y/n]” y
and then it was printed
“Reading old configuration” …
After upgrade complete, old configuration of MT-box was kept.
BUT:
Next step was upgrade from 3.27 to 3.28, from ISO, too.
Again, I was also asked,
“Do you want to save old configuration [y/n]” y
BUT then the disk was formatted, WITHOUT “Reading old configuration …”, which was correct, however, because all configuration data was lost 
So I assume, any upgrade from 3.27 or lower to 3.30 using ISO will also loose the configuration. So be on guard.