Thank you for your suggestion.
I fixed the problem but I have a new one.
The analysis runs perfectly when I define the nonlinear sprigs in this way:
*Spring, elset=Springs/Dashpots-1-spring,nonlinear
-10000, -2
-5000, -1
0, 0
5000, 1
10000, 2
Instead I get this error:
"***ERROR...