"There is a very negative time step detected" warnings

Quite regularly (e.g. reported by @Nikos in the last general meeting), shower simulations throw the warning:

There is a very negative time step detected: -1.42207e-17 s. This is not physical and may easily crash subsequent modules. Set to 0_s, but CHECK AND FIX.

From a first debugging, this happens especially for low-energy electrons/positrons. Furthermore, the warning seems to vanish if one turns of multiple scattering, so this seems to be related.

However, I don't yet understand why this is related, because next_intersect() (where the error is thrown) is called from getTrack(), but getTrack() is called in Cascade.inl before the continuous processes (and therefore multiple scattering) is applied.