See Prime 6 in attach (one pendulum with a ribbon)
Looks like you forgot to attach the file!
BTW, how could L(F):=L.0 be a solution? Given the function L seen in your screenshot this seems to be not possible. F=0 is the only value for F where L(F) would be L.0. Or did you mean something else?
Strange and quite sensible. Usually this could mean "check your equations"?
See here the result with three different end-values and notice the magnitude of the force values.
Force seems to be rising and positive in these scenarios but the error thrown ("number with magnitude greater than 10^307) when calculating L(F(t)) would only occur if F goes below -1413 ?????