personal pronoun

every time I write thank you in an academic letter it tells me 

"Review this sentence for personal pronouns                   The personal pronoun, “you”, may not be appropriate for formal writing.  Consider removing this pronoun, and rephrasing your sentence.       Formal writing should be impersonal, so personal pronouns – possessive or otherwise- are generally not used.  Personal pronouns (i.e. I, you, we, my, mine, your, yours, our, ours) assume the information in your writing applies only to specific readers.  By using impersonal pronouns (he, she, one, they, his, him, her, one’s, their), any reader may make their own personal connections to the information being discussed.         Hint: replace you, I and we with one, and replace my, mine, yours and ours with one’s."

my sentence was 

Dr. Cohen thank you for your input and explaining the process.

Any ideas ? Thanks :)