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.
return '{tablename___theotherelement_raw}';
By the way... are you really using J! 2.5 and an old, old Fabrik? Or shouldn't you post in the "Fabrik 3.1+ for Joomla 3.0+ Community" forum?
Well, unless you're using a custom list template anyway, I can't think of a quicker and simpler solution than using a calc element.