The July 2026 update was released July 14, 2026. This introduced many security fixes but this can cause issues with your SAML, OIDC, or FBA implementations. This can prevent tokens from being added or read from dcache.
Here are some logs from my 2019 farm:
07/18/2026 12:18:03.74 w3wp.exe (0x0FF8) 0x1914 SharePoint Foundation Security Token Handler ayreb Unexpected Session cookie transform was triggered unexpectedly during encode. This path should not be reached in normal operation. To disable this protection, set the farm property ‘SessionCookieTransformProtectionEnabled‘ to false. 908628a2-2a7c-10e3-545f-1f5de6df25d3
07/18/2026 12:18:03.74 w3wp.exe (0x0FF8) 0x1914 SharePoint Foundation DistributedCache air38 Medium Token Cache: Failed to add token from distributed cache. Key: ‘0e.t|microsoftentratrust|user1@TestTestTest.onmicrosoft.com’, Exception: ‘System.NotSupportedException: Specified method is not supported. at Microsoft.SharePoint.IdentityModel.SPTokenCache.NotSupportedCookieTransform.Encode(Byte[] value) at Microsoft.IdentityModel.Tokens.SessionSecurityTokenHandler.ApplyTransforms(Byte[] cookie, Boolean outbound) at Microsoft.IdentityModel.Tokens.SessionSecurityTokenHandler.WriteToken(XmlWriter writer, SecurityToken token) at Microsoft.IdentityModel.Tokens.SecurityTokenSerializerAdapter.WriteTokenCore(XmlWriter writer, SecurityToken token) at Microsoft.SharePoint.Administration.Claims.SPTokenCacheEntry.SerializeToken(SecurityTokenSerializer tokenSerializer) at Microsoft.SharePoint.IdentityModel.SPDistributedSecurityTokenCache.Put(String key, Object value) at Microsoft.SharePoint.IdentityModel.SPTokenCache.AddCachedTokenV1(String cachekey, SPTokenCacheEntry value)’. 908628a2-2a7c-10e3-545f-1f5de6df25d3
07/18/2026 12:18:03.74 w3wp.exe (0x0FF8) 0x1914 SharePoint Foundation DistributedCache air39 Medium Token Cache: Reverting to local cache to Add the token. Key: ‘0e.t|microsoftentratrust|user1@TestTestTest.onmicrosoft.com’. 908628a2-2a7c-10e3-545f-1f5de6df25d3
The key here is to disable SessionCookieTranformProtectionEnabled which is new for July 2026. This will only work on SharePoint 2016 and 2019.
$farm = Get-SPFarm
$farm.Properties[“SessionCookieTransformProtectionEnabled”] = $false;
$farm.Update()
IISReset #all serversSharePoint SE is currently investigating the issue above and hoping to get a fix. I would suggest using sticky sessions or persistence at your NLB. Failing to read a token from the dcache will fall back to the local cache(the STS). If the request hits a different WFE than the one you and the token is unable to be localed(dcache and local), SharePoint invalidates the clients FedAuth cookie and makes them reauthenicate.
Log entry from SPSE:
07/23/2026 14:34:31.14 w3wp.exe (0x6128) 0x2814 SharePoint Foundation DistributedCache aa6e6 Unexpected Unexpected error in ByteArrayToObject – Exception ‘System.NotSupportedException: Specified method is not supported. at Microsoft.SharePoint.IdentityModel.SPSessionSecurityTokenHandlerV2.ReadToken(XmlReader reader) at System.IdentityModel.Tokens.SecurityTokenHandlerCollection.ReadToken(XmlReader reader) at Microsoft.SharePoint.Administration.Claims.SPTokenCacheEntry..ctor(SerializationInfo info, StreamingContext context) at ReadSPTokenCacheEntryFromXml(XmlReaderDelegator , XmlObjectSerializerReadContext , XmlDictionaryString[] , XmlDictionaryString[] ) at System.Runtime.Serialization.ClassDataContract.ReadXmlValue(XmlReaderDelegator xmlReader, XmlObjectSerializerReadContext context) at System.Runtime.Serialization.XmlObjectSerializerReadContext.InternalDeserialize(XmlReaderDelegator reader, String name, String ns, Type declaredType, DataContract& dataContract) at System.Runtime.Serialization.XmlObjectSerializerReadContext.InternalDeserialize(XmlReaderDelegator xmlReader, Type declaredType, DataContract dataContract, String name, String ns) at System.Runtime.Serialization.DataContractSerializer.InternalReadObject(XmlReaderDelegator xmlReader, Boolean verifyObjectName, DataContractResolver dataContractResolver) at System.Runtime.Serialization.XmlObjectSerializer.ReadObjectHandleExceptions(XmlReaderDelegator reader, Boolean verifyObjectName, DataContractResolver dataContractResolver) at System.Runtime.Serialization.XmlObjectSerializer.ReadObject(XmlDictionaryReader reader) at Microsoft.SharePoint.DistributedCaching.SPDistributedCacheUtils.ByteArrayToObject[T](Byte[] Buffer)’.