Several cancellations on this route this morning owing to what RTT stated was, in each case, a "Planning Error". Out of interest, what is the term likely to mean in this case? Bus replacements laid on for the morning peak, and possibly beyond.
Looking at the advanced view on RTT, it gives the delay code as TI, which is listed as "Traincrew rostering problem" in the Delay Attribution Guide (full list of codes here). In my experience when TI is used it's typically announced as "a shortage of available train crew" or similar, so it appears Southern were/are particularly short of drivers (or possibly guards) this morning.