I'm trying to solve a system of DAEs with discrete conditions:

muk = 0.2; mus = 0.3; m1 = 1; m2 = 2; m3 = 3; Fn12 = 3; Fn23 = 2; minvel = 0.1; F1[x_] := 2*Sin[5*x]; F2[x_] := 2*Sin[7*x]; Fs12[x_] := (m2*F1[x] - m1*F2[x])/(m1 + m2); Off[NDSolve::pdord] s = NDSolve[{m1*x1''[t] == F1[t] - Ff12[t], x1'[0] == 0, x1[0] == 0, m2*x2''[t] == F2[t] + Ff12[t], x2'[0] == 0, x2[0] == 0, Ff12[t] == If[Abs[x1''[t] - x2''[t]] < 0.1 && Abs[Fs12[t]] < mus*Fn12 , Fs12[t], muk*Fn12*Sign[x1''[t] - x2''[t]]]}, {x1, x2, Ff12}, {t, 10}] 

However when solving I get two weird errors

NDSolve::tddisc

NDSolve cannot do a discontinuity replacement for event surfaces that depend only on time

NDSolve::nderr

Error test failure at t==...; unable to continue

which searching on google does not offer that much of the results!

I have found some relevant posts and documentations (one, two, three). It seems that I have to use a helper function or something like that using Piecewise and/or WhenEvent functions but I can't get my head around it!

I would appreciate if you could explain why this error happens and how I can modify my code to avoid it?