Hi Michael,
I think this could come from compilation options incompatibility between late ROOT version (>6.24 I believe) and CASToR. CASToR still forces compilation with c++11, while ROOT uses more recent norm (14 or 17). It could be related to something else as well though.
So I would first try to recompile CASToR with CXXFLAGS / CMAKE_CXX_FLAGS set to a more recent norm and check if that solves the issue.
Best,
Thibaut