SItecore Identity Server on Prem throws" Error: An error occured"

In my case after removing the custom configuration of Publish:end event resolved the issue.

    <events>
  <event name="publish:end">
    <handler type="Sitecore.Publishing.HtmlCacheClearer, Sitecore.Kernel" method="ClearCache">
        ....
        ....
     </handler>
  </event> </events>

If using SPE you can clear up all or even for a specific user. I'm using Sitecore 9.3 and still see this issue happening.

Import-Function -Name Invoke-SqlCommand

$connection = [Sitecore.Configuration.Settings]::GetConnectionString("core")

$query = @"
DELETE FROM [dbo].[Properties]
WHERE dbo.Properties.[Key] LIKE 'CORE_SC_TICKET%'
AND dbo.Properties.[Value] LIKE '%mwest%'
"@
Invoke-SqlCommand -Connection $connection -Query $query

I am facing the same issue with Sitecore 9.1, in summary the issue is Sitecore not being able to handle long ticket in Core database

Sitecore has a cleanup task that should take care of cleaning the tickets in the database but for some reason it is not performing, and if you could check your logs and see if you have something similar

ManagedPoolThread #2 12:04:16 INFO Job started: Sitecore.Tasks.CleanupAuthenticationTicketsAgent ManagedPoolThread #2 12:04:16 INFO CleanupAuthenticationTicketsAgent: Number of expired authentication tickets that have been removed: 0 ManagedPoolThread #2 12:04:16 INFO CleanupAuthenticationTicketsAgent: Total number of authentication tickets to process: 0 ManagedPoolThread #2 12:04:16 INFO Job ended: Sitecore.Tasks.CleanupAuthenticationTicketsAgent (units processed: )

OR

20892 10:35:42 ERROR String or binary data would be truncated. The statement has been terminated. Exception: System.Exception Message: String or binary data would be truncated. The statement has been terminated. Source: Sitecore.Kernel at Sitecore.Data.DataProviders.Sql.DataProviderCommand.ExecuteNonQuery() at Sitecore.Data.DataProviders.Sql.SqlDataApi.<>c__DisplayClass33_0.b__0() at Sitecore.Data.DataProviders.NullRetryer.Execute[T](Func`1 action, Action recover) at Sitecore.Data.Properties.SqlPropertyStore.SetStringValueCore(String prefixedName, String value) at Sitecore.Data.Properties.PropertyStore.SetStringValue(String name, String value) at Sitecore.Web.Authentication.DefaultTicketManager.CreateTicket(String userName, String startUrl, Boolean persist) at Sitecore.Owin.Authentication.Pipelines.CookieAuthentication.SignedIn.CreateTicket.Process(SignedInArgs args) at (Object , Object )

Nested Exception

Exception: System.Data.DataException Message: Error executing SQL command: INSERT INTO [Properties] ( [Key], [Value] ) VALUES ( @name, @value )

I hope you are able to find similar entries, however, if that's not the case, please give a shot using the following command in your SQL against the Core database as a workaround and please note this is going to kill ALL sessions connected to CM

DELETE FROM [yourCoreDatabase].[dbo].[Properties] WHERE [Key] LIKE '%CORE_SC_TICKET%'

I am currently working with Sitecore Support since it seems that was fixed on Sitecore 9.0.1 and Sitecore 9.0.2 but newer versions are still facing such issue.

References: https://viniciusdeschamps.com.br/error-an-error-occurred-after-logging-in-sitecore/

Although the message "an error occured” is of course useless, I have found that when this occurs, there are also errors logged in the Sitecore logs. Check those.