The formula E=mc^2 is correct, but Einstein’s proof of it is incorrect. I had this feeling for a long time, but never wrote a clear argument why this must be the case. Now I wrote a paper that shows it beyond any doubt, and the paper is really simple
https://www.researchgate.net/publication/355809133_The_error_in_Einstein’s_Emc2_derivation
If you wonder why it is so easy for me to show incorrect well known results in theoretical physics, it is not any big mystery: the field was partially taken over in Germany around Einstein’s time and totally taken over around the Second World War when the center of science moved to the USA from war-ridden Europe. It was the 1930s immigrants who made US science the top of the world, but it was not any real top science, only it seems to be as critical views were suppressed and all these improbable results, like Einstein’s and Feynman’s and so on, were accepted. Now everybody believes them, but just check yourself. Now that field (and many others) is totally taken over: all efforts to show the errors are simply ignored. These taken over fields are full of junk and people try not to notice it.