diff --git a/app/src/main/kotlin/org/koitharu/kotatsu/core/parser/MangaLoaderContextImpl.kt b/app/src/main/kotlin/org/koitharu/kotatsu/core/parser/MangaLoaderContextImpl.kt
index 195bb9dab..815290238 100644
--- a/app/src/main/kotlin/org/koitharu/kotatsu/core/parser/MangaLoaderContextImpl.kt
+++ b/app/src/main/kotlin/org/koitharu/kotatsu/core/parser/MangaLoaderContextImpl.kt
@@ -16,6 +16,7 @@ import org.koitharu.kotatsu.core.network.cookies.MutableCookieJar
 import org.koitharu.kotatsu.core.prefs.SourceSettings
 import org.koitharu.kotatsu.core.util.ext.configureForParser
 import org.koitharu.kotatsu.core.util.ext.printStackTraceDebug
+import org.koitharu.kotatsu.core.util.ext.sanitizeHeaderValue
 import org.koitharu.kotatsu.core.util.ext.toList
 import org.koitharu.kotatsu.parsers.MangaLoaderContext
 import org.koitharu.kotatsu.parsers.config.MangaSourceConfig
@@ -41,7 +42,7 @@ class MangaLoaderContextImpl @Inject constructor(
 	private val userAgentLazy = SuspendLazy {
 		withContext(Dispatchers.Main) {
 			obtainWebView().settings.userAgentString
-		}
+		}.sanitizeHeaderValue()
 	}
 
 	@SuppressLint("SetJavaScriptEnabled")
diff --git a/app/src/main/kotlin/org/koitharu/kotatsu/core/prefs/SourceSettings.kt b/app/src/main/kotlin/org/koitharu/kotatsu/core/prefs/SourceSettings.kt
index 482de71a9..f6c2f4d28 100644
--- a/app/src/main/kotlin/org/koitharu/kotatsu/core/prefs/SourceSettings.kt
+++ b/app/src/main/kotlin/org/koitharu/kotatsu/core/prefs/SourceSettings.kt
@@ -3,9 +3,11 @@ package org.koitharu.kotatsu.core.prefs
 import android.content.Context
 import android.content.SharedPreferences.OnSharedPreferenceChangeListener
 import androidx.core.content.edit
+import okhttp3.internal.isSensitiveHeader
 import org.koitharu.kotatsu.core.util.ext.getEnumValue
 import org.koitharu.kotatsu.core.util.ext.ifNullOrEmpty
 import org.koitharu.kotatsu.core.util.ext.putEnumValue
+import org.koitharu.kotatsu.core.util.ext.sanitizeHeaderValue
 import org.koitharu.kotatsu.parsers.config.ConfigKey
 import org.koitharu.kotatsu.parsers.config.MangaSourceConfig
 import org.koitharu.kotatsu.parsers.model.MangaSource
@@ -25,7 +27,10 @@ class SourceSettings(context: Context, source: MangaSource) : MangaSourceConfig
 	@Suppress("UNCHECKED_CAST")
 	override fun <T> get(key: ConfigKey<T>): T {
 		return when (key) {
-			is ConfigKey.UserAgent -> prefs.getString(key.key, key.defaultValue).ifNullOrEmpty { key.defaultValue }
+			is ConfigKey.UserAgent -> prefs.getString(key.key, key.defaultValue)
+				.ifNullOrEmpty { key.defaultValue }
+				.sanitizeHeaderValue()
+
 			is ConfigKey.Domain -> prefs.getString(key.key, key.defaultValue).ifNullOrEmpty { key.defaultValue }
 			is ConfigKey.ShowSuspiciousContent -> prefs.getBoolean(key.key, key.defaultValue)
 			is ConfigKey.SplitByTranslations -> prefs.getBoolean(key.key, key.defaultValue)
@@ -36,7 +41,7 @@ class SourceSettings(context: Context, source: MangaSource) : MangaSourceConfig
 		when (key) {
 			is ConfigKey.Domain -> putString(key.key, value as String?)
 			is ConfigKey.ShowSuspiciousContent -> putBoolean(key.key, value as Boolean)
-			is ConfigKey.UserAgent -> putString(key.key, value as String?)
+			is ConfigKey.UserAgent -> putString(key.key, (value as String?)?.sanitizeHeaderValue())
 			is ConfigKey.SplitByTranslations -> putBoolean(key.key, value as Boolean)
 		}
 	}
diff --git a/app/src/main/kotlin/org/koitharu/kotatsu/core/util/ext/Http.kt b/app/src/main/kotlin/org/koitharu/kotatsu/core/util/ext/Http.kt
index 988e06920..a95e7508b 100644
--- a/app/src/main/kotlin/org/koitharu/kotatsu/core/util/ext/Http.kt
+++ b/app/src/main/kotlin/org/koitharu/kotatsu/core/util/ext/Http.kt
@@ -1,11 +1,13 @@
 package org.koitharu.kotatsu.core.util.ext
 
 import okhttp3.Cookie
+import okhttp3.Headers
 import okhttp3.HttpUrl
 import okhttp3.MediaType.Companion.toMediaType
 import okhttp3.RequestBody.Companion.toRequestBody
 import okhttp3.Response
 import okhttp3.internal.closeQuietly
+import okhttp3.internal.isSensitiveHeader
 import okio.IOException
 import org.json.JSONObject
 import org.jsoup.HttpStatusException
@@ -59,3 +61,16 @@ fun Cookie.newBuilder(): Cookie.Builder = Cookie.Builder().also { c ->
 		c.httpOnly()
 	}
 }
+
+fun String.sanitizeHeaderValue(): String {
+	return if (all(Char::isValidForHeaderValue)) {
+		this // fast path
+	} else {
+		filter(Char::isValidForHeaderValue)
+	}
+}
+
+private fun Char.isValidForHeaderValue(): Boolean {
+	// from okhttp3.Headers$Companion.checkValue
+	return this == '\t' || this in '\u0020'..'\u007e'
+}
diff --git a/app/src/main/kotlin/org/koitharu/kotatsu/settings/storage/directories/MangaDirectoriesActivity.kt b/app/src/main/kotlin/org/koitharu/kotatsu/settings/storage/directories/MangaDirectoriesActivity.kt
index d4f3283f9..7e2d35eec 100644
--- a/app/src/main/kotlin/org/koitharu/kotatsu/settings/storage/directories/MangaDirectoriesActivity.kt
+++ b/app/src/main/kotlin/org/koitharu/kotatsu/settings/storage/directories/MangaDirectoriesActivity.kt
@@ -13,13 +13,16 @@ import androidx.core.graphics.Insets
 import androidx.core.view.isVisible
 import androidx.core.view.updateLayoutParams
 import androidx.core.view.updatePadding
+import com.google.android.material.snackbar.Snackbar
 import com.hannesdorfmann.adapterdelegates4.AsyncListDifferDelegationAdapter
 import dagger.hilt.android.AndroidEntryPoint
+import org.koitharu.kotatsu.R
 import org.koitharu.kotatsu.core.exceptions.resolve.SnackbarErrorObserver
 import org.koitharu.kotatsu.core.ui.BaseActivity
 import org.koitharu.kotatsu.core.ui.list.OnListItemClickListener
 import org.koitharu.kotatsu.core.util.ext.observe
 import org.koitharu.kotatsu.core.util.ext.observeEvent
+import org.koitharu.kotatsu.core.util.ext.tryLaunch
 import org.koitharu.kotatsu.databinding.ActivityMangaDirectoriesBinding
 import org.koitharu.kotatsu.settings.storage.DirectoryDiffCallback
 import org.koitharu.kotatsu.settings.storage.DirectoryModel
@@ -42,7 +45,11 @@ class MangaDirectoriesActivity : BaseActivity<ActivityMangaDirectoriesBinding>()
 	) {
 		if (it) {
 			viewModel.updateList()
-			pickFileTreeLauncher.launch(null)
+			if (!pickFileTreeLauncher.tryLaunch(null)) {
+				Snackbar.make(
+					viewBinding.recyclerView, R.string.operation_not_supported, Snackbar.LENGTH_SHORT,
+				).show()
+			}
 		}
 	}
 
@@ -68,7 +75,11 @@ class MangaDirectoriesActivity : BaseActivity<ActivityMangaDirectoriesBinding>()
 	}
 
 	override fun onClick(v: View?) {
-		permissionRequestLauncher.launch(Manifest.permission.WRITE_EXTERNAL_STORAGE)
+		if (!permissionRequestLauncher.tryLaunch(Manifest.permission.WRITE_EXTERNAL_STORAGE)) {
+			Snackbar.make(
+				viewBinding.recyclerView, R.string.operation_not_supported, Snackbar.LENGTH_SHORT,
+			).show()
+		}
 	}
 
 	override fun onWindowInsetsChanged(insets: Insets) {