how do I resolve this system singular error?

Question

This is a question about futuremice.

I am trying to run the mice MI in parallel. This is my code.

imp_df <- futuremice(df, 
                     pred = pred_matrix, 
                     meth = imp_method, 
                     n.core= 20, 
                     maxit = 20, 
                     m=20, 
                     print = TRUE,
                     parallelseed = 123,
                     progress = TRUE,
                     remove.collinear = FALSE,
                     nnet.MaxWts= 6000)

I am getting an error, Caused by error in solve.default: ! Lapack routine dgesv:system is exactly singular: U[603, 603] = 0

However when I switch to regular mice

        imp_df <- mice(df, 
                     pred = pred_matrix, 
                     meth = imp_method, 
                     n.core= 20, 
                     maxit = 20, 
                     m=20, 
                     print = TRUE,
                     seed = 123,
                     progress = TRUE,
                    # remove.collinear = FALSE,
                     nnet.MaxWts= 6000)

This error disappears, I dont see this error. Any suggestions where I am going wrong with futuremice ? Thanks.