In skins_fallback_v6\_item_block.inc.php on line 107
// We want to display the post time:
$Item->issue_time( array(
'before' => ' '.T_('posted on '),
'after' => ' ',
'time_format' => 'M j, Y',
) );
The T_('posted on ') has a blank space after 'on'. Just remove it then it's OK!
（Or maybe the line "posted on" in .po file should be added a blank space after the word 'on'）
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b2evolution CMS Support Forums
You're right @haharen, the solution is to get rid of the blank space in the code.
Actually, it's already solved and the fix will be included in the next release (https://github.com/b2evolution/b2evolution/blob/release/6.6.x/skins_fallback_v6/_item_block.inc.php#L107).
Thank you.