Neither was right. BODYholic was right.
误解=misunderstand some abstract thoughts, concepts.
错认为=wrongly recognize as
Also you can say: 我常常被 误会是/误会成 德国人，但是我其实是法国人啊。(Notice you missed a 是 in the second clause.)
Maybe you'd like to learn the meaning of seperate characters: 误 mistake, wrong, wrongly; 解 explain, explanation; 错 wrong; 认 recognize; 为 as; 会 understand; 是 is。