The study considers mathematical problem solving to be at the heart of mathematics teaching and learning, while mathematical challenge is a core element of any educational process. The study design addresses the complexity of teachers' knowledge. It is aimed at exploring the development of teachers' mathematical and pedagogical conceptions associated with systematic employment of multiple-solution tasks (MSTs) in a "problem-solving" course for prospective mathematics teachers (PMTs). Our attention to teachers' mathematical conceptions focused on the development of PMTs' problem-solving competences. Our attention to teachers' meta-mathematical and pedagogical conceptions focused on changes in teachers' views concerning the level of interest and level of difficulty of the mathematical tasks. We differentiated between the systematic and craft modes of professional development integrated in the course. Systematic mode involved problem-solving sessions and reflective discussions on collective solution spaces. Craft mode involved interviewing school students. The study demonstrates the effectiveness of MSTs for PMTs' professional development.