I do not have any experience per say as a Project Manager, however I can give my perspective as a Scrum Master and what I have seen throughout my time helping in leading teams.
There are 2 main areas when it comes to my current and past relationships between the Project Manager and Scrum Master role.
Team Issues:
When working on a project you will be working with people and with that you will have lots of personalities. All teams need someone to help lead, and that may require some amount of management. This is especially true if issues are causing a high amount of incompatibility among the team. These disagreements, and difference of opinions, could end up having a high impact on the progress of the project. Facilitation is key here!
Scope Creep:
This may seem obvious, but sometimes the simplest things to solve can be the most complex. However, this is probably one of the biggest challenges that a Project Manager will face during a project (Scrum Masters included). It is extremely important to act on this when it does happen and take the appropriate steps in order to refocus to the new direction.
This is from an outside looking in perspective.