It is related to the number of atoms that may be subjected to the sbound force. How many water oxygens do you have within the SBOUND sphere? The default size of the SBOUND table should be OK for a ~40A radius sphere.

For help with input files: Always submit a minimal file (the shortest possible!!!) that leads to the problem you observe.
Lennart Nilsson
Karolinska Institutet
Stockholm, Sweden