Go to user setting under that user and check the signature setting,
If the screen show error.
Go to Mailnow directory --> users --> domain folder -->username
Edit setting xml on signature part like below, make sure the properties is 1100 not 0100 or 0110.
<WebmailSignature>
<Setting>
<Name>Webmail Sign Text</Name>
<Value>ffdd</Value>
<Properties>1100</Properties>
</Setting>
</WebmailSignature>