How did you set $g_normal_date_format and $g_datetime_picker_format ?

Did you consider the note and the warning at https://www.mantisbt.org/docs/master/en-US/Admin_Guide/html-desktop/#admin.config.date ?

hmmm

$g_short_date_format    = 'd-m-Y';
$g_normal_date_format   = 'd-m-Y H:i';
$g_complete_date_format = 'd-m-Y H:i T';

$g_datetime_picker_format does not exist
Keep in mind, i configured these way back when in 2007. Did not even know that they could be configured... It's just that long ago
Might have been interesting to implement a check in the install.php on upgrades for users where this differs from the default...

But case closed as far as i'm concerned. Thanks