diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5f69ae66..1a7c2fa4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@
 ## v24.20
 
 ### Pre-releases
+- `v24.20-alpha14`
 - `v24.20-alpha13`
 - `v24.20-alpha12`
 - `v24.20-alpha11`
@@ -23,6 +24,7 @@
 - Default password criteria are more restrictive (#372, `v24.20-alpha1`, Compatible with Seacat Auth Webui v24.19-alpha and later, Seacat Account Webui v24.08-beta and later)
 
 ### Fix
+- Fix nginx oauth introspection for self-encoded (algorithmic) sessions (#405, `v24.20-alpha14`)
 - Fix token request for self-encoded (algorithmic) sessions (#404, `v24.20-alpha13`)
 - Fix credential search performance (#391, `v24.20-alpha12`)
 - Fix AttributeError in credentials update (#399, `v24.20-alpha11`)
diff --git a/seacatauth/cookie/service.py b/seacatauth/cookie/service.py
index ffe44d64..26dedc83 100644
--- a/seacatauth/cookie/service.py
+++ b/seacatauth/cookie/service.py
@@ -8,7 +8,6 @@
 import asab
 import asab.storage
 import asab.exceptions
-import jwcrypto.jws
 
 from .. import exceptions
 from ..session.adapter import SessionAdapter, CookieData
@@ -106,17 +105,9 @@ async def get_session_by_session_cookie_value(self, cookie_value: str):
 		"""
 		Get session by cookie value.
 		"""
-		# First try interpreting the token as an algorithmic session
 		if "." in cookie_value:
-			try:
-				return await self.SessionService.Algorithmic.deserialize(cookie_value)
-			except asab.exceptions.NotAuthenticatedError as e:
-				# The JWToken is invalid or expired
-				raise exceptions.SessionNotFoundError(
-					"Invalid algorithmic session token", query={"cookie_value": cookie_value}) from e
-			except jwcrypto.jws.InvalidJWSObject:
-				# Not a JWT token
-				pass
+			# If there is ".", the value is not pure base64. It must be a JWT of an algorithmic session.
+			return await self.SessionService.Algorithmic.deserialize(cookie_value)
 
 		# Then try looking for the session in the database
 		try:
diff --git a/seacatauth/openidconnect/service.py b/seacatauth/openidconnect/service.py
index 767cce35..452d8b80 100644
--- a/seacatauth/openidconnect/service.py
+++ b/seacatauth/openidconnect/service.py
@@ -552,6 +552,10 @@ async def get_session_by_access_token(self, token_value: str):
 		"""
 		Retrieve session by its access token.
 		"""
+		if "." in token_value:
+			# If there is ".", the value is not pure base64. It must be a JWT of an algorithmic session.
+			return await self.SessionService.Algorithmic.deserialize(token_value)
+
 		try:
 			token_bytes = base64.urlsafe_b64decode(token_value.encode("ascii"))
 		except binascii.Error as e:
diff --git a/seacatauth/session/algorithmic.py b/seacatauth/session/algorithmic.py
index e62d5797..3ecc2d4f 100644
--- a/seacatauth/session/algorithmic.py
+++ b/seacatauth/session/algorithmic.py
@@ -7,12 +7,11 @@
 import uuid
 import json
 import datetime
-
-import asab.exceptions
 import asab.web.rest
 import asab.metrics
 
 from .adapter import SessionAdapter
+from .. import exceptions
 from ..authz import build_credentials_authz
 
 L = logging.getLogger(__name__)
@@ -114,12 +113,14 @@ async def deserialize(self, token_value) -> SessionAdapter | None:
 		"""
 		try:
 			token = jwcrypto.jwt.JWT(jwt=token_value, key=self.PrivateKey)
-		except ValueError:
-			# This is not a JWToken
-			return None
-		except (jwcrypto.jws.InvalidJWSSignature, jwcrypto.jwt.JWTExpired) as e:
-			# JWToken invalid
-			raise asab.exceptions.NotAuthenticatedError() from e
+		except (ValueError, jwcrypto.jws.InvalidJWSObject) as e:
+			L.error("Corrupt algorithmic session token.")
+			raise exceptions.SessionNotFoundError("Corrupt algorithmic session token.") from e
+		except jwcrypto.jws.InvalidJWSSignature as e:
+			L.error("Invalid algorithmic session token signature.")
+			raise exceptions.SessionNotFoundError("Invalid algorithmic session token signature.") from e
+		except jwcrypto.jwt.JWTExpired as e:
+			raise exceptions.SessionNotFoundError("Expired algorithmic session token.") from e
 
 		data_dict = json.loads(token.claims)
 		client_dict = await self.ClientService.get(data_dict["azp"])
@@ -130,7 +131,10 @@ async def deserialize(self, token_value) -> SessionAdapter | None:
 				client_dict=client_dict,
 				scope=data_dict["scope"])
 		except Exception as e:
-			raise asab.exceptions.NotAuthenticatedError() from e
+			L.error(
+				"Failed to build session from algorithmic session token claims.", struct_data=data_dict)
+			raise exceptions.SessionNotFoundError(
+				"Failed to build session from algorithmic session token claims.") from e
 
 		return session