For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
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//Force the "Please select" value to integer; user element extends dbjoin which has '' as default value
$d->options[0]->value = (int)$d->options[0]->value;
I can't replicate.This is only an issue when the field is HIDDEN.
Which version of user and databasejoin plugin?
$d = $displayData;
if (!isset($d->value) || empty($d->value) ) $d->value = 0;
Is the element in a group created by a list join?