What happened? Sitecore 10 was running fine with its fresh install, you deployed your existing code from your Sitecore 8 instance to do your upgrade, that all worked, and now you start setting up your UAT or Production instance and you get nearly a blank screen. All it says is "an error occurred" [sic].
Loading up Sitecore worked, going through the login process with Identity Server worked... until you get redirected back to Sitecore.
There used to be an SC_TICKET error that would cause this in Sitecore 9, but it was patched. What's going on now?
This is what's happening. It used to be that Sitecore required you to set up cache clearing yourself. Lots of people would have a patch file like this:
<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
<sitecore>
<events timingLevel="custom">
<event name="publish:end">
<handler type="Sitecore.Publishing.HtmlCacheClearer, Sitecore.Kernel" method="ClearCache">
<sites hint="list">
<site name="MySite">MySite</site>
</sites>
</handler>
</event>
</events>
</sitecore>
</configuration>
If you check the logs, you'll see errors referring to this.
27700 17:45:56 ERROR Could not resolve type name: Sitecore.Publishing.HtmlCacheClearer, Sitecore.Kernel (method: Sitecore.Configuration.DefaultFactory.CreateFromTypeName(XmlNode configNode, String[] parameters, Boolean assert)). 24712 17:45:56 ERROR Could not resolve type name: Sitecore.Publishing.HtmlCacheClearer, Sitecore.Kernel (method: Sitecore.Configuration.DefaultFactory.CreateFromTypeName(XmlNode configNode, String[] parameters, Boolean assert)). ManagedPoolThread #5 17:45:57 INFO Trying to load XML configuration /App_Config/Security/GlobalRoles.config ManagedPoolThread #6 17:45:57 INFO Trying to load XML configuration /App_Config/Security/Domains.config ManagedPoolThread #5 17:45:57 ERROR Could not resolve type name: Sitecore.Publishing.HtmlCacheClearer, Sitecore.Kernel (method: Sitecore.Configuration.DefaultFactory.CreateFromTypeName(XmlNode configNode, String[] parameters, Boolean assert)).
I know it doesn't look like it has anything to do with login, but it does. (It actually breaks a lot of processes, but login is the one you'll notice first.)
The Good News
You don't need this setup anymore! Sitecore has handled this cache clearing event as part of its standard installation now. Just delete the cache configuration and move on.
It might take a bit of work going through all the different environment setups, but its one less thing to maintain.
P.S. This will actually happen in Sitecore 9 as well, and the same solution applies.
