A Headway- or Timetable-based assignment or a calculation of PuT skim matrices leads to a program crash ("Runtime Error") or takes more runtime than expected.
Possibly too many links are open for a transport system of type "PuTWalk". The runtime error is caused by lack of memory. The PuT assignment procedures are not intended to assign "pedestrians". "PuTWalk" is intended for connectors and transfer paths which are not modelled inside a stop. The PuT assignment procedure assembles every single link as a path leg into the search graph. Too many foot links inflate the search graph, which causes a runtime error by lack of memory. Of course, that does not only cost memory capacity but also calculation time. For that reason, better use foot paths only where necessary.
Use (long) connectors instead. It might be a set of connectors with a dedicated type number, factorized for their Walk times, to model that people walk though some subordinated network and not an air-line distance.