Online learning platforms take over a new role in education, especially in times of the COVID-19 pandemic. This paper will discuss pre-service teachers' reflections on attitudes towards online learning platforms and the respective changes due to an online training of using this digital tool in mathematics classes. The special training took place in a bachelor seminar with fourteen participants on the use of digital media in mathematics education at the University of Siegen. Based on the 'Tripartite Model of Attitude Structure' which defines the psychological concept of attitude, data material about the pre-service teachers' attitudes is gathered by pre- and post-reflection-questionnaires. A qualitative content analysis led to the formation of a system of six main categories and approximately 51 descriptors for pre-service teachers' attitudes towards online learning platforms and especially their changes due to the online training of using these platforms. The descriptors can be a basis for further research studies on this topic.