for the benefit of complete and detailed information, the warnings were all these:
PHP Notice: Undefined index: excludeLastDays in /var/www/w/extensions/FlaggedRevs/FlaggedRevsSetup.php on line 78
PHP Notice: Undefined index: excludeLastDays in /var/www/w/extensions/FlaggedRevs/FlaggedRevsSetup.php on line 93
PHP Notice: Undefined index: totalCheckedEdits in /var/www/w/extensions/FlaggedRevs/FlaggedRevsSetup.php on line 97
PHP Notice: Undefined index: excludeLastDays in /var/www/w/extensions/FlaggedRevs/FlaggedRevsSetup.php on line 98
PHP Notice: Undefined index: maxRevertedEditRatio in /var/www/w/extensions/FlaggedRevs/FlaggedRevsSetup.php on line 101
PHP Notice: Undefined index: neverBlocked in /var/www/w/extensions/FlaggedRevs/FlaggedRevsSetup.php on line 104
but thanks to your clue, I found out that in /var/www/w/extensions/FlaggedRevs/ there is beautiful README file that reads like this:
In 1.34 the autopromote config was removed from the default. If you want to keep the same config
add the following to your LocalSettings.php
$wgFlaggedRevsAutopromote = [
'days' => 60, # days since registration
'edits' => 250, # total edit count
'excludeLastDays' => 1, # exclude the last X days of edits from below edit counts
'benchmarks' => 15, # number of "spread out" edits
'spacing' => 3, # number of days between these edits (the "spread")
'totalContentEdits' => 300, # edits to pages in $wgContentNamespaces
'totalCheckedEdits' => 200, # edits before the stable version of pages
'uniqueContentPages' => 14, # unique pages in $wgContentNamespaces edited
'editComments' => 50, # number of manual edit summaries used
'userpageBytes' => 0, # size of userpage (use 0 to not require a userpage)
'neverBlocked' => true, # username was never blocked before?
'maxRevertedEditRatio' => 0.03, # max fraction of edits reverted via "rollback"/"undo"
I did changed my LocalSettings.php and now everything is fine with FR.
Perhaps the installation/configuration instructions deserve some attention and some updates.
Thank you so much @Ciencia Al Poder