diff --git a/src/libslic3r/GCode/ToolOrdering.cpp b/src/libslic3r/GCode/ToolOrdering.cpp
index ee03d6e959..65c887f343 100644
--- a/src/libslic3r/GCode/ToolOrdering.cpp
+++ b/src/libslic3r/GCode/ToolOrdering.cpp
@@ -138,7 +138,8 @@ static double calc_max_layer_height(const PrintConfig &config, double max_object
 {
     double max_layer_height = std::numeric_limits<double>::max();
     for (size_t i = 0; i < config.nozzle_diameter.values.size(); ++ i) {
-        double mlh = config.max_layer_height.values[i];
+        // max_layer_height may be shorter than the extruder count; get_at() clamps.
+        double mlh = config.max_layer_height.get_at(i);
         if (mlh == 0.)
             mlh = 0.75 * config.nozzle_diameter.values[i];
         max_layer_height = std::min(max_layer_height, mlh);
diff --git a/src/slic3r/GUI/ConfigManipulation.cpp b/src/slic3r/GUI/ConfigManipulation.cpp
index 00e2cebc44..edae8c6c79 100644
--- a/src/slic3r/GUI/ConfigManipulation.cpp
+++ b/src/slic3r/GUI/ConfigManipulation.cpp
@@ -946,7 +946,7 @@ void ConfigManipulation::toggle_print_fff_options(DynamicPrintConfig *config, in
     toggle_line("wipe_tower_extra_rib_length", have_rib_wall);
     toggle_line("wipe_tower_rib_width", have_rib_wall);
     toggle_line("wipe_tower_fillet_wall", have_rib_wall);
-    toggle_field("prime_tower_width", have_prime_tower && (supports_wipe_tower_2 || have_rib_wall));
+    toggle_field("prime_tower_width", have_prime_tower && !have_rib_wall);
 
     toggle_line("single_extruder_multi_material_priming", !bSEMM && have_prime_tower && supports_wipe_tower_2);
 
diff --git a/src/slic3r/GUI/GUI_App.cpp b/src/slic3r/GUI/GUI_App.cpp
index 121acdb947..feaddf07ab 100644
--- a/src/slic3r/GUI/GUI_App.cpp
+++ b/src/slic3r/GUI/GUI_App.cpp
@@ -6965,6 +6965,10 @@ void GUI_App::start_sync_user_preset(bool with_progress_dlg)
             // finishFn tears down the progress dialog (and clears the re-entrancy guard), so it
             // must run on every exit path — otherwise an early bail-out would leak the modal
             // dialog and leave the guard stuck, blocking all later manual syncs.
+            // Guard the whole thread body: an uncaught exception here (e.g. a transient
+            // boost::filesystem error while scanning the preset folder) would otherwise
+            // propagate out of the thread and terminate the entire application.
+            try {
             if (!m_agent) { finishFn(false); return; }
 
             // One-time scan for orphaned .info files left over from offline deletions; queues HTTP DELETEs.
@@ -7206,6 +7210,11 @@ void GUI_App::start_sync_user_preset(bool with_progress_dlg)
                     boost::this_thread::sleep_for(boost::chrono::milliseconds(500));
                 }
             }
+            } catch (const std::exception& e) {
+                BOOST_LOG_TRIVIAL(error) << "user preset sync thread terminated by exception: " << e.what();
+            } catch (...) {
+                BOOST_LOG_TRIVIAL(error) << "user preset sync thread terminated by unknown exception";
+            }
         });
 }
 
@@ -8526,8 +8535,13 @@ void GUI_App::scan_orphaned_info_files()
         if (!fs::exists(type_dir))
             continue;
 
-        // Iterate through all .info files
-        for (auto& entry : boost::filesystem::directory_iterator(type_dir)) {
+        // Iterate through all .info files. Use the error_code-based iterator so a transient
+        // directory-read failure (e.g. macOS readdir returning ENOTSUP) is logged and skipped
+        // instead of throwing an uncaught exception that would terminate the app from the
+        // background sync thread this runs on.
+        boost::system::error_code ec;
+        for (boost::filesystem::directory_iterator it(type_dir, ec), end; !ec && it != end; it.increment(ec)) {
+            const auto& entry = *it;
             if (entry.path().extension() != ".info")
                 continue;
 
@@ -8546,6 +8560,8 @@ void GUI_App::scan_orphaned_info_files()
                 }
             }
         }
+        if (ec)
+            BOOST_LOG_TRIVIAL(warning) << "scan_orphaned_info_files: failed to scan " << type_dir.string() << ": " << ec.message();
     }
 }
 
diff --git a/tests/fff_print/test_multifilament.cpp b/tests/fff_print/test_multifilament.cpp
index 12c1cae766..f33a8e1e61 100644
--- a/tests/fff_print/test_multifilament.cpp
+++ b/tests/fff_print/test_multifilament.cpp
@@ -85,3 +85,22 @@ TEST_CASE("Per-object wall filament override is honored", "[MultiFilament]")
     CHECK(tools_for_role(gcode, "perimeter") == std::set<int>{ 0, 1 });
     CHECK(tools_for_role(gcode, "infill")    == std::set<int>{ 0 }); // infill not overridden: stays on F1
 }
+
+// max_layer_height can be shorter than the extruder count (normalization sizes it to the
+// filament count under single_extruder_multi_material). calc_max_layer_height() in ToolOrdering
+// indexed it per-nozzle and read past the end. Shortened directly here to isolate that read;
+// the other per-extruder keys stay extruder-length so slicing reaches the code under test.
+TEST_CASE("Multi-extruder slice stays in bounds with a short max_layer_height", "[MultiFilament]")
+{
+    DynamicPrintConfig config = multifilament_config(2);
+    config.set_deserialize_strict({
+        { "nozzle_diameter",           "0.4,0.4" },
+        { "printer_extruder_id",       "1,2" },
+        { "printer_extruder_variant",  "Direct Drive Standard,Direct Drive Standard" },
+        { "extruder_printable_height", "0,0" },
+        { "max_layer_height",          "0.3" }, // deliberately one entry short
+    });
+    Print print;
+    init_and_process_print({ cube(20) }, print, config);
+    REQUIRE_FALSE(print.objects().front()->layers().empty());
+}