The User Code Execution Request was refused because the User Code Execution Host Service was too busy to handle the request

While activating the sandbox solution from 21apps . It seems that several requirements are required for sandboxed solutions to work:

Only upload solutions with *.wsp extentions
When the solution hasn't a wsp extension but for example a zip extension, the activate button remains grayed out.

Always check if the User Code Service is running
Even if you are 100% certain it was activated yesterday or the day before 😉

The Sandbox service has been renamed in the RTM to Microsoft SharePoint Foundation Sandboxed Code Service !!!

Run powershell script when Domain Controller is installed on same server
I received an error (The User Code Execution Request was refused because the User Code Execution Host Service was too busy to handle the request). It seems that a powershell script is required in order to make this work.

$acl = Get-Acl HKLM:\System\CurrentControlSet\Control\ComputerName 
$person = [System.Security.Principal.NTAccount]"Users"
$access = [System.Security.AccessControl.RegistryRights]::FullControl
$inheritance = [System.Security.AccessControl.InheritanceFlags]"ContainerInherit, ObjectInherit"
$propagation = [System.Security.AccessControl.PropagationFlags]::None
$type = [System.Security.AccessControl.AccessControlType]::Allow
$rule = New-Object System.Security.AccessControl.RegistryAccessRule($person, $access, $inheritance, $propagation, $type)
$acl.AddAccessRule($rule)
Set-Acl HKLM:\System\CurrentControlSet\Control\ComputerName $acl

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