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.
Is this when deleting a file?
What is the max number of files set to for AJAX upload?
-- hugh
Hmm, how recent is your Fabrik install?
The line number you gave me in the error (3111) is in the middle of the onDeleteRows method, only called when you delete row(s) in the list and have elected to delete associated files in the upload element when rows are deleted.
https://github.com/Fabrik/fabrik/blob/master/plugins/fabrik_element/fileupload/fileupload.php#L3111
-- hugh