I had the issue, that I was not able to sign a database on a server with the Server ID after upgrading the client to R9.0.1.

After opening a PMR ( 76570,010,618 ) I received the information from Lotus Support:

On further investigation I found a similar customer report where an admin was having problems signing databases since upgrading the Admin client to 9.0.1.

The issue was found to be caused by the admin having his own name in there personal address book, my contacts and recent contacts. When removed, the issue was addressed.                                  
                                                                       
Please check the following:                                                
– Open your contacts names.nsf and remove your own name from my contacts and from recent contacts and restart the Notes client.                                              
– Afterwards see if can you sign databases successfully.