Adding to a task list - User does not exist or is not unique

Question

We have SharePoint 2010 with a single web server front end and another server as the SQL server.  When trying to add a task to a list or an announcement to a page, users are getting the error, "The user does not exist or is not unique."
I have searched diligently for an answer, but cannot find anything about this with regards to users getting the error. 
Thank you in advance for your assistance.

Answer 1

hello,
i have faced your problem few minutes ago. when i was trying to solve another problem
--->> (Object Cache: The super user account utilized by the cache is not configured) --> my current problem
i have found at search results the below PS command to configure super user and super reader.
$w = Get-SPWebApplication http://webapplicationlink
$w.Properties["portalsuperuseraccount"] = "domain account"
$w.Properties["portalsuperreaderaccount"] = "domain account"
$w.Update()
but it caused me another problem which you are facing right now.
---> ( Error:  The user does not exist or is not unique)
while adding any item to any list specially lists
then i have tried to rollback by doing the below PS command and it worked and solved it.
$w = Get-SPWebApplication http://webapplicationlink
$w.Properties["portalsuperuseraccount"] = ""
$w.Properties["portalsuperreaderaccount"] = ""
$w.Update()
after that check for the super user and super reader by
Get-SPWebApplication | %{Write-Host "Web Application: " $_.url "`nSuper user: " $_.properties["portalsuperuseraccount"] "`nSuper reader: " $_.properties["portalsuperreaderaccount"] "`n"}
This is the output for the above command should be like this.
Web Application:  http://webapplicationlink
Super user:
Super reader:
hope that it helps :)
Karim El Zarka,Senior IT Specialist Commercial International Bank, MCTS Sharepoint