Abstract In this study, constraint-based argumentation scaffolding was proposed to facilitate online argumentation performance and ill-structured problem solving during online discussions. In addition, epistemological beliefs were presumed to play a role in solving ill-structured diagnosis–solution problems. Constraint-based discussion boards were implemented to scaffold pre-service teachers' online discussions about behaviour management (diagnosis–solution) problems. The scaffolded discussion group generated more evidence notes and also generated more hypothesis messages and hypothesis testing messages as well as problem space construction messages. There was a relationship between epistemological beliefs and ill-structured problem solving. Simple knowledge, omniscient authority, and fixed ability significantly predicted problem-solving performance. A significant negative relationship between simple knowledge and individual problem-solving performance was found. This implies that individuals who believe in simple knowledge may be less inclined to explore more solution alternatives. However, contrary to prediction, omniscient authority and fixed ability beliefs were positively associated with problem-solving processes.