It crashes with a memory corruption error (invalid free), maybe caused by an unhandled exception or otherwise side effect of destruction of the stack objects. Just an assumption based on the code comments in the method causing the crash:
mitk::PropertyPersistenceInfo::UseRegEx
This is during startup, so some plugin loading through CTK and Qt and dlopen, so I suspect there was some change in Qt 5.15 which changed the behaviour.
I did not yet do any further debugging, but using Qt 5.12 on an otherwise identical setup does work