The line search error usually happens when the computed log prob or gradient have some error and the search direction does not point downhill. ODE solver is iterative and has its own tolerance affecting the accuracy. In your case it seems that the optimization is very near the optimum as log prob and ||grad|| are not changing much, and ||dx|| is small. Very small alpha (related to step size in line search) in Iter 191 supports the assumption that the search direction is not pointing downhill.
See “Optimization-Specific Commands” in CmdStan user guide
You can also test running with smaller ODE solver tolerances whether there is any practical difference in the optimum found.