He was accused of plagiarism at the time? Isn't that calculation sorta simple?

PS: no, he was not accused of plagiarism at the time.

Abraham Pais says that Soldner's paper was entirely unknown to the physics community until 1921, when Nobelist Philipp Lenard reproduced it in an attempt to discredit Einstein, and prevent him from winning a Nobel Prize. Apparently, Lenard was a Nazi, and critical of "non-German" science. Still, that is years later, after Einstein had already changed the calculations.