The success of Business Process Management (BPM) projects heavily depends on effective collaboration. Specifically, collaboration with actors from outside an organization (such as customers, suppliers, or distributors) is becoming increasingly important. Despite the significance of this phenomenon, little research has yet been done to systematically investigate into the potential obstacles that organizations might encounter when engaging in such types of BPM projects. Therefore, we apply a qualitative study of three in-depth case studies in order to explore obstacles to BPM collaboration with firm-external actors. As a result, we offer the following contributions: Theory-wise, we develop a theoretical framework for studying emergent collaboration phenomena and elaborate the nature of collaboration in BPM by introducing three "collaboration idealtypes" which specify the set of actors involved. Then, we present an empirically grounded overview of potential obstacles to collaborative BPM projects that involve firm-external actors. Finally, we lay out elements of a problem-oriented theory which contributes to addressing challenges and to harvesting the potential benefits of collaboration in BPM projects.