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
Please see our announcement here.
ok, yes.
List, form, details are displaying "W" correctly if displaying existing dates (This is done via PHP and therefore working)
But a click on the calendar widget inserts V instead of week number, it seems JS doesn't know "W" even with "Advanced formats" enabled.
Workaround: use W only in list format