diff --git a/backend/open_webui/routers/users.py b/backend/open_webui/routers/users.py
index d07cdf745d..b3dc5a9991 100644
--- a/backend/open_webui/routers/users.py
+++ b/backend/open_webui/routers/users.py
@@ -701,6 +701,10 @@ async def reset_all_users_interface_settings(
     Clears the 'ui' key in all users' settings. Admin only.
     """
     try:
+        # Match dialects explicitly. Falling through to SQLite-specific SQL
+        # (json_set / json_extract) for every non-postgres backend would
+        # silently fail on MySQL / MariaDB / other engines with an opaque
+        # syntax error instead of a clear "unsupported dialect" response.
         dialect = db.bind.dialect.name
 
         if dialect == 'postgresql':
@@ -723,10 +727,10 @@ async def reset_all_users_interface_settings(
             )
             result = await db.execute(stmt)
             reset_count = result.rowcount
-        else:
-            # SQLite: use native json_set (available since SQLite 3.9+).
-            # rowcount on the UPDATE result reflects the rows actually
-            # changed, so no follow-up SELECT changes() round-trip is needed.
+        elif dialect == 'sqlite':
+            # Native json_set is available since SQLite 3.9+. rowcount on the
+            # UPDATE result reflects the rows actually changed, so no
+            # follow-up SELECT changes() round-trip is needed.
             result = await db.execute(
                 text(
                     """
@@ -738,6 +742,14 @@ async def reset_all_users_interface_settings(
                 )
             )
             reset_count = result.rowcount
+        else:
+            raise HTTPException(
+                status_code=status.HTTP_501_NOT_IMPLEMENTED,
+                detail=(
+                    f"Resetting interface settings is not implemented for the '{dialect}' "
+                    'database dialect; supported dialects are postgresql and sqlite.'
+                ),
+            )
 
         await db.commit()
 
diff --git a/src/lib/components/admin/Settings/Interface/InterfaceDefaultsModal.svelte b/src/lib/components/admin/Settings/Interface/InterfaceDefaultsModal.svelte
index 8c9f1dbf81..c853065bb4 100644
--- a/src/lib/components/admin/Settings/Interface/InterfaceDefaultsModal.svelte
+++ b/src/lib/components/admin/Settings/Interface/InterfaceDefaultsModal.svelte
@@ -1,6 +1,6 @@
 <script lang="ts">
 	import { toast } from 'svelte-sonner';
-	import { getContext } from 'svelte';
+	import { getContext, tick } from 'svelte';
 
 	import Modal from '$lib/components/common/Modal.svelte';
 	import Spinner from '$lib/components/common/Spinner.svelte';
@@ -48,6 +48,19 @@
 	const submitHandler = async () => {
 		saving = true;
 		try {
+			// The embedded InterfaceSettings form has submit-scoped writes
+			// (updateInterfaceHandler -> saveSettings({ models, imageCompressionSize }))
+			// that don't fire on per-control change. Request-submit the child
+			// form first so those fields land in adminDefaults via the
+			// saveAdminSettings callback, then wait one tick for reactivity
+			// to flush before we POST. Without this, admins could save
+			// defaults and silently lose submit-only fields.
+			const innerForm = document.getElementById('tab-interface') as HTMLFormElement | null;
+			if (innerForm && typeof innerForm.requestSubmit === 'function') {
+				innerForm.requestSubmit();
+			}
+			await tick();
+
 			await setInterfaceDefaults(localStorage.token, prepareForBackend(adminDefaults));
 			toast.success($i18n.t('Interface defaults saved successfully'));
 			show = false;
diff --git a/src/routes/+layout.svelte b/src/routes/+layout.svelte
index 165465eafa..df5512f3e4 100644
--- a/src/routes/+layout.svelte
+++ b/src/routes/+layout.svelte
@@ -935,7 +935,27 @@
 				// defaults is that THOSE users get them, so always merge
 				// against adminDefaults instead of skipping straight to
 				// localStorage when userSettings is falsy.
-				const localStorageSettings = JSON.parse(localStorage.getItem('settings') ?? '{}');
+				//
+				// Guard the localStorage read: a stale or hand-edited
+				// localStorage.settings value can be invalid JSON, and
+				// letting JSON.parse throw here would abort the whole
+				// authenticated-load path even when we already have valid
+				// userSettings from the backend.
+				let localStorageSettings: any = {};
+				if (!userSettings?.ui) {
+					const raw = localStorage.getItem('settings');
+					if (raw) {
+						try {
+							localStorageSettings = JSON.parse(raw) ?? {};
+						} catch (parseErr) {
+							console.warn(
+								'Ignoring malformed localStorage.settings during init:',
+								parseErr
+							);
+							localStorageSettings = {};
+						}
+					}
+				}
 				const userUI = userSettings?.ui ?? localStorageSettings ?? {};
 				settings.set(deepMerge(adminDefaults ?? {}, userUI));
 				setTextScale($settings?.textScale ?? 1);