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.
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This is not fabrik, but a Notice shouldn't hurt.yougrids/yjsgcore/yjsg_core.php
There are a lot of threads about this bug, update from GitHub (no Joomla update).Class 'uploader' not found