From March 2020 until June 2021 secondary education in Greece experienced the consequences of the COVID-19 pandemic. The first measure taken was the general closure of face-to-face teaching at all levels of education and the provision of distance learning through the so-called emergency remote teaching. Mathematicians dealt with unprecedented and difficult situations. In the present paper we record the views of 218 mathematicians of secondary education working in Greek public schools about the obstacles they faced in their teaching during the pandemic in a quantitative survey. According to the survey connectivity problems (affecting teachers and students), lack of teaching material suitable for distance learning, unsatisfactory support of the state, poor previous training in distance learning were major issues. This is the first survey monitoring the problems that Greek mathematicians faced during this period, how they dealt with them and one of a few worldwide regarding mathematicians.