We need setting aa initial condition:

.ic v(out20)=0

examples/Monte_Carlo/MC_ring.sp

@@ -33,6 +33,9 @@ xinv11 dd 0 sub well out buf inv1
 cout  buf ss 0.2pF
 *
 .options noacct
+* for convergence with KLU
+.ic v(out20) = 0
+
 .control
   save buf                        $ we just need buf, save memory by more than 10x
   let mc_runs = 30             $ number of runs for monte carlo

examples/Monte_Carlo/mc_ring_circ.net

@@ -41,6 +41,9 @@ xinv5 dd ss sub well out20 out inv5
 xinv11 dd 0 sub well out buf inv1
 cout  buf ss 0.2pF
 
+* for convergence with KLU
+.ic v(out20) = 0
+
  *** Model library files.
 * Add your library here
 * Chose the transistors for XMP1 and XMN1 accordingly