This warning indicates a low fill level of a connection matrix of a SparseConnection. This means that a SparseConnection has reserver a much larger buffer than needed. When you initialize a connection with random sparse connectivity, usually SparseConnection tries to make a good guess about how much memory is needed before filling the matrix. However, particularly for small values for size_pre x size_post this estimate can be off. For larger matrices this is more likely to indicate a problem with your code. If you manually connect your connection in the simulation program then you probably know what a low fill level means for you.

Make sure everything is in order!