After upgrading Sitefinity CMS to version 15.4.8633, all frontend page requests served by the ASP.NET Core Renderer fail with a NullReferenceException. The CMS backend (/Sitefinity) and REST API (/restapi/health) remain operational with no infrastructure issues.

The Renderer encounters ErrorCodeException errors in the _MetaInfo.cshtml partial view when communicating with the CMS via the REST SDK, and the CMS throws exceptions in CustomOutputCacheVariationsRegistry.GetCachabilityRestriction.

The issue only occurs when the Redis output cache provider is used. The issue is not reproduced when using the InMemory output cache provider.

This issue occurs specifically when upgrading from version 15.4.8631 to 15.4.8634 while using the Redis output cache provider. The Redis cache retains cached entries from the previous version, and the incompatibility between the old and new cache entry format triggers the error.

Environments using the InMemory output cache provider are not affected because InMemory cache does not persist entries across application restarts. The cache is cleared automatically when the application starts with the new version.

The issue is caused by a bug in Sitefinity.

Until the fix is available, use the workaround below.

Either of the following options can be applied as a workaround:

Flush the Redis cache:

Flush the Redis cache to remove the stale serialized entries left behind after the upgrade. No code change or configuration update is required. Once the cache is cleared, the application rebuilds it cleanly and requests are served normally.

1. Flush the Redis cache.
   • For Sitefinity Cloud: Contact Sitefinity Support to request a Redis cache flush. 
   • For on-premises: Clear the Redis cache using your Redis management tool or the Redis CLI FLUSHDB command.
2. Verify that all frontend pages load without errors.

Switch the output cache provider to InMemory:

If flushing the Redis cache is not immediately possible, temporarily switch the output cache provider from Redis to InMemory.

1. Navigate to Administration > Settings > Advanced.
2. Expand System > OutputCacheSettings.
3. Change the defaultOutputCacheProvider value from Redis to InMemory.
4. Save the changes and restart the application.

For Sitefinity Cloud environments, add the following application setting to the Custom AppSettings variable group in the Azure DevOps library:

• Key: sf-env:systemConfig/outputCacheSettings:defaultOutputCacheProvider
• Value: InMemory

Note: InMemory output caching may not provide the same performance characteristics as Redis, particularly under high traffic. This is a temporary measure. Once the upgrade to 15.4.8635 or later has been applied or the Redis cache has been flushed, switch the output cache provider back to Redis and remove the InMemory override from the variable group if applicable.