In my class of writing, we are doing a paper. We are required to avoid fragments, run-ons, comma splices, no-parallelism, incorrect punctuation, etc. 

I wrote this sentence: 

"Even though some researchers are against authentic materials because they might be time-consuming, complex, and too culturally based; most of the studies have stated that learners “who were trained with authentic materials, maintained positive attitudes more than those who did not get a similar training.” (Varmis Kilic & Genc Ilter, 2015)."

My teacher told me it has an error, but I can't identify it. Could you please help me out with this one?