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This PHP warning is related to the new update of WordPress (5.3) which had a change in Walker class.
Previously the walker method had 2 arguments. In the new update, it has more arguments.
In order to fix the issue, You need to change the code from Post_Terms_Order_Walker::walk($elements, $max_depth) to Post_Terms_Order_Walker::walk($elements, $max_depth, ...$args)
Note: Possibly the rests of errors are caused by the first one. As it is going to output the error, it sends headers.