diff --git a/pypesto/optimize/ess/ess.py b/pypesto/optimize/ess/ess.py
index 8dab32f56..4323e207f 100644
--- a/pypesto/optimize/ess/ess.py
+++ b/pypesto/optimize/ess/ess.py
@@ -528,10 +528,15 @@ def _do_local_search(
                 f"{optimizer_result.exitflag}: {optimizer_result.message}"
             )
             if np.isfinite(optimizer_result.fval):
-                self.local_solutions.append(optimizer_result.x)
+                local_solution_x = optimizer_result.x[
+                    optimizer_result.free_indices
+                ]
+                local_solution_fx = optimizer_result.fval
+
+                self.local_solutions.append(local_solution_x)
 
                 self._maybe_update_global_best(
-                    optimizer_result.x, optimizer_result.fval
+                    local_solution_x, local_solution_fx
                 )
                 break
 
diff --git a/pypesto/optimize/ess/sacess.py b/pypesto/optimize/ess/sacess.py
index 3dfb68cae..30177533d 100644
--- a/pypesto/optimize/ess/sacess.py
+++ b/pypesto/optimize/ess/sacess.py
@@ -311,7 +311,9 @@ def reconfigure_worker(self, worker_idx: int) -> Dict:
             ].copy()
             for setting in ["local_n2", "balance", "dim_refset"]:
                 if setting in leader_options:
-                    self._ess_options[worker_idx] = leader_options[setting]
+                    self._ess_options[worker_idx][setting] = leader_options[
+                        setting
+                    ]
             return self._ess_options[worker_idx].copy()
 
     def submit_solution(
@@ -356,8 +358,14 @@ def submit_solution(
                     f"Accepted solution from worker {sender_idx}: {fx}."
                 )
                 # accept
+                if len(x) != len(self._best_known_x):
+                    raise AssertionError(
+                        f"Received solution with {len(x)} parameters, "
+                        f"but expected {len(self._best_known_x)}."
+                    )
+                for i, xi in enumerate(x):
+                    self._best_known_x[i] = xi
                 self._best_known_fx.value = fx
-                self._best_known_x.value = x
                 self._worker_comms[sender_idx] += 1
                 self._worker_scores[sender_idx] = (
                     self._worker_comms[sender_idx] * elapsed_time_s
@@ -543,10 +551,14 @@ def _cooperate(self):
             f"(known best: {self._best_known_fx}).",
         )
         if recv_fx < self._best_known_fx or (
-            not np.isfinite(self._best_known_fx) and np.isfinite(recv_x)
+            not np.isfinite(self._best_known_fx) and np.isfinite(recv_fx)
         ):
+            if not np.isfinite(recv_x).all():
+                raise AssertionError(
+                    f"Received non-finite parameters {recv_x}."
+                )
             self._logger.debug(
-                f"Worker {self._worker_idx} received better solution."
+                f"Worker {self._worker_idx} received better solution {recv_fx}."
             )
             self._best_known_fx = recv_fx
             self._n_received_solutions += 1
@@ -577,7 +589,7 @@ def maybe_update_best(self, x: np.array, fx: float):
         self._logger.debug(
             f"Worker {self._worker_idx} maybe sending solution {fx}. "
             f"best known: {self._best_known_fx}, "
-            f"rel change: {(self._best_known_fx - fx) / fx:.4g}, "
+            f"rel change: {(fx - self._best_known_fx) / fx:.4g}, "
             f"threshold: {self._acceptance_threshold}"
         )