I have tracked this down to be a version mismatch of the support library FastMM_FullDebugMode.dll.
An older version of the library works with the newer version compiled into the executable. There seems to be no check that versions do match. However, modules don't really work together at run-time.
I think it is wise to add a version check so FastMM ensures the DLL is of the same version (or maybe a compatible version, but that makes version checking logic more cumbersome).
From https://stackoverflow.com/questions/22461755/does-calling-fastmm4-logallocatedblockstofile-periodically-use-up-memory-space:
I think it is wise to add a version check so FastMM ensures the DLL is of the same version (or maybe a compatible version, but that makes version checking logic more cumbersome).