Just an update: tried to run the same but with the original oblique data and additional to the standard

+ WARNING: If you are performing spatial transformations on an oblique dset, such as /path/roi_sphere_30_21_18_flirt.nii.gz,or viewing/combining it with volumes of differing obliquity, you should consider running: 3dWarp -deoblique on this and other oblique datasets in the same session.

there is still the error of

Matrix error: Incompatible dimensions for vector multiplication: 0x0 X 1
Memory usage: chunks=42 bytes=619956



so it doesn't seem that deobliquing caused this problem!