diff --git a/telethon/errors/__init__.py b/telethon/errors/__init__.py
index f6bc16e5..ebb6ed07 100644
--- a/telethon/errors/__init__.py
+++ b/telethon/errors/__init__.py
@@ -6,7 +6,7 @@ import re
 
 from .common import (
     ReadCancelledError, TypeNotFoundError, InvalidChecksumError,
-    InvalidBufferError, SecurityError, CdnFileTamperedError,
+    InvalidBufferError, AuthKeyNotFound, SecurityError, CdnFileTamperedError,
     AlreadyInConversationError, BadMessageError, MultiError
 )
 
diff --git a/telethon/errors/common.py b/telethon/errors/common.py
index de7d95f8..4f2573d6 100644
--- a/telethon/errors/common.py
+++ b/telethon/errors/common.py
@@ -1,5 +1,6 @@
 """Errors not related to the Telegram API itself"""
 import struct
+import textwrap
 
 from ..tl import TLRequest
 
@@ -58,6 +59,22 @@ class InvalidBufferError(BufferError):
                 'Invalid response buffer (too short {})'.format(self.payload))
 
 
+class AuthKeyNotFound(Exception):
+    """
+    The server claims it doesn't know about the authorization key (session
+    file) currently being used. This might be because it either has never
+    seen this authorization key, or it used to know about the authorization
+    key but has forgotten it, either temporarily or permanently (possibly
+    due to server errors).
+
+    If the issue persists, you may need to recreate the session file and login
+    again. This is not done automatically because it is not possible to know
+    if the issue is temporary or permanent.
+    """
+    def __init__(self):
+        super().__init__(textwrap.dedent(self.__class__.__doc__))
+
+
 class SecurityError(Exception):
     """
     Generic security error, mostly used when generating a new AuthKey.
diff --git a/telethon/network/mtprotosender.py b/telethon/network/mtprotosender.py
index 05fd39e9..80c0c528 100644
--- a/telethon/network/mtprotosender.py
+++ b/telethon/network/mtprotosender.py
@@ -10,7 +10,7 @@ from .mtprotostate import MTProtoState
 from ..tl.tlobject import TLRequest
 from .. import helpers, utils
 from ..errors import (
-    BadMessageError, InvalidBufferError, SecurityError,
+    BadMessageError, InvalidBufferError, AuthKeyNotFound, SecurityError,
     TypeNotFoundError, rpc_message_to_error
 )
 from ..extensions import BinaryReader
@@ -377,11 +377,8 @@ class MTProtoSender:
             except BufferError as e:
                 # TODO there should probably only be one place to except all these errors
                 if isinstance(e, InvalidBufferError) and e.code == 404:
-                    self._log.info('Broken authorization key; resetting')
-                    self.auth_key.key = None
-                    if self._auth_key_callback:
-                        await self._auth_key_callback(None)
-
+                    self._log.info('Server does not know about the current auth key; the session may need to be recreated')
+                    last_error = AuthKeyNotFound()
                     ok = False
                     break
                 else:
@@ -521,12 +518,8 @@ class MTProtoSender:
                 continue
             except BufferError as e:
                 if isinstance(e, InvalidBufferError) and e.code == 404:
-                    self._log.info('Broken authorization key; resetting')
-                    self.auth_key.key = None
-                    if self._auth_key_callback:
-                        await self._auth_key_callback(None)
-
-                    await self._disconnect(error=e)
+                    self._log.info('Server does not know about the current auth key; the session may need to be recreated')
+                    await self._disconnect(error=AuthKeyNotFound())
                 else:
                     self._log.warning('Invalid buffer %s', e)
                     self._start_reconnect(e)