TYPO3 Core

new, fresh installation of 8.5.0 under Windows (XAMPP) environment:

Webserver:    Apache/2.4.23 (Win32) OpenSSL/1.0.2h PHP/7.0.9
PHP Version:    7.0.9
Database (Default):    MySQL 5.5.5

after the installation, open the install tool > section "Upgrade Wizard"
(please note the the installation is new, so there is nothing to update) > the progress
bar is at 95% and it reads the following message:

Update database schema: Modify tables and fields

There are tables or fields in the database which need to be changed.

This update wizard can be run only when there are no other update wizards left to make sure they have all needed fields unchanged.

If I click the "execute" button:

Update database schema: Modify tables and fields

Change the following keys in tables:

sys_file_processedfile: identifier
sys_file_reference: tablenames_fieldname
sys_category_record_mm: uid_foreign_tablefield

selecting "Show database queries performed" and clicking "Perfom updates!"

DROP INDEX `identifier` ON sys_file_processedfile 
CREATE INDEX `identifier` ON sys_file_processedfile (storage, identifier(199)) 
DROP INDEX `tablenames_fieldname` ON sys_file_reference 
CREATE INDEX `tablenames_fieldname` ON sys_file_reference (tablenames(32), fieldname(12)) 
DROP INDEX `uid_foreign_tablefield` ON sys_category_record_mm 
CREATE INDEX `uid_foreign_tablefield` ON sys_category_record_mm (uid_foreign, tablenames(40), fieldname(3), sorting_foreign) 

the update seems successful (it appears the message "Update successful"), but if I go back to the wizard, the progress bar is still
at 95% and it asks to perform the same update .

Notes:

1) According to Install tool > important actions > database analyzer, the database is OK

2)if I try for example to perform

CREATE INDEX `identifier` ON sys_file_processedfile on phpmyadmin

I get the error

1.Unexpected beginning of statement. (near "sys_file_processedfile" at position 29)

3) executing

SELECT @@SQL_MODE, @@GLOBAL.SQL_MODE;

on phpmyadmin, the result is:

NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION