How do I do this?   "check the Directory Server "WebSTS" Windows event log"

On the server hosting Directory Server:

1. Open Event Viewer
2. Expand Applications and Services Logs
3. Expand Laserfiche
4. Expand Directory Service
5. Expand WebSTS (LFDSSTS logs go here, LFDS service logs go under Server)
6. Look at the Admin and Operations logs. One of them, likely the Admin log, will have an entry starting with the error text you saw on the page ("An error has occurred") followed by the stack trace. This is the "full error message" I'm looking for. You can use the timestamps on the log entries to help identify the correct message. 
7. Please post the text that appears in the General tab's box (see below). If there are multiple different error messages, especially if they occur back-to-back timewise, please include both.

This is useful troubleshooting advice for Laserfiche applications in general. Most serious errors are written to the application event logs with more details than you get in the UI. Reading an error's stack trace until you find a line that helps make sense of it (example) is perhaps the single most useful tool you can have in your Laserfiche troubleshooting toolbelt.

Here are some recent logs.

Admin:

An error has occurred

STS.Core.Infrastructure.Exceptions.WebSTSException: An error has occurred ---> System.Exception: Unable to log in: could not identify Laserfiche Directory Server site for user login.
   at WebSTS.LFDS.Services.Login.LFDSLoginManager.ProcessLogin(LoginData loginData)
   at WebSTS.LFDS.Controllers.LoginController.Login(LoginData data)
   at lambda_method(Closure , Object , Object[] )
   at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.<>c__DisplayClass10.<GetExecutor>b__9(Object instance, Object[] methodParameters)
   at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ExecuteAsync(HttpControllerContext controllerContext, IDictionary`2 arguments, CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Controllers.ApiControllerActionInvoker.<InvokeActionAsyncCore>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Controllers.ActionFilterResult.<ExecuteAsync>d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Controllers.ExceptionFilterResult.<ExecuteAsync>d__0.MoveNext()
 

==========================

Operation:

ID4243: Could not create a SecurityToken. A token was not found in the token cache and no cookie was found in the context.

System.IdentityModel.Tokens.SecurityTokenException: ID4243: Could not create a SecurityToken. A token was not found in the token cache and no cookie was found in the context.
   at System.IdentityModel.Tokens.SessionSecurityTokenHandler.ReadToken(XmlReader reader, SecurityTokenResolver tokenResolver)
   at System.IdentityModel.Tokens.SessionSecurityTokenHandler.ReadToken(Byte[] token, SecurityTokenResolver tokenResolver)
   at System.IdentityModel.Services.SessionAuthenticationModule.ReadSessionTokenFromCookie(Byte[] sessionCookie)
   at System.IdentityModel.Services.SessionAuthenticationModule.TryReadSessionTokenFromCookie(SessionSecurityToken& sessionToken)
   at System.IdentityModel.Services.SessionAuthenticationModule.OnAuthenticateRequest(Object sender, EventArgs eventArgs)
   at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

"Unable to log in: could not identify Laserfiche Directory Server site for user login." could suggest the STS web app does not have a valid connection to the LFDS service.

Can you go to https://localhost/LFDSSTS/configuration on the machine hosting STS and see if it reports any errors in the UI? If it doesn't, try changing any setting and selecting "Update" to see if that throws errors (if not, go ahead and change the setting back).