Background: Open Source Software (OSS) is often seen as an option to mitigate risks of lock-ins. Yet, single-vendor OSS can still result in soft lock-ins due to knowledge asymmetries and technical barriers. Aim: This study explores actors that render such soft lock-ins. Research design: We conduct a qualitative case study of an E-service Platform (ESP) used by over 190+ municipalities. Results: User-driven lock-in factors emerged as a significant category, including limited and non-transparent communication, restrictive qualification requirements in procurement, confusion on maintainership, and comfort in the status quo. Technical lock-in factors include inadequate documentation, dependency management issues, and limited test coverage. Conclusions: Strong leadership and continuous training is needed to address presence of comfort and conservative culture among municipalities. Open Source Stewards, i.e., neutral hosts for OSS projects, can support municipalities in these tasks while also helping to foster an open, competitive collaboration that can enable a broader supplier ecosystem.