@Thomas Richard Thanks for the suggestions. The Initialisation Diagnostics app looks interesting, but everything seemed fine in it. The Solver Diagnostics gave some more information, returning this:

(in DSN/RunSimulation) condition iteration failure: unable to solve Tarjan form at initial point: attempting to compute radical of a negative/zero value during solution of linear 1-block (block 53) for variable diff(`Main.SSContact_1.friction3_1.z_lim`(t),t), radical: 1/`dsnr/abs`(`Main.SSContact_1.AWF2.AFInputX.Fx`(t)^2+`Main.SSContact_1.AWF2.AFInputY.Fy`(t)^2+`Main.SSContact_1.AWF2.AFInputZ.Fz`(t)^2)^(1/2)

I was able to fix the problem by changing the input of one of my blocks to an equivalent input, though I am uncertain I understand the original problem. Does `dsnr/abs` mean anything special? Is it just the 'abs' function?