OK, finally I found out which php.ini file is in use here for my PHP7.1 installation :-p and retried the tiki-installer process and now it seems all worked fine after I changed the value from Off to
display_errors = On. The error_reporting value I have here is
error_reporting = E_ALL & ~E_DEPRECATED & ~E_STRICT
Now instead of the tiki-installer asking me for db user credentials in a loop it seems the upgrade button worked:
Your database has been configured and Tiki is ready to run! You can now log in into Tiki as user admin and start configuring the application.
Upgrade operations executed successfully: 18128 SQL queries."
Weirdly I have seen no PHP errors reported in the process...
Yep, I have nicely upgraded Tiki 18.1 from 17.x: https://demo.luciash.eu/tiki17xto18x/HomePage