Symptoms

Some users getting the following error message when accessing K2 smartforms:

Exception System.Web.HtpunhandledExeption was thrown
Argument 'userFqn' maynot be null or empty
Parameter name userFqn

It seems that only users from one specific additional domain are affected.
 

Diagnoses

Normally in such scenario you have to verify your multi-domain configuration. Please refer to the following documentation for details on multi-domain configurations:

1) Verify that domain has been added/configured correctly from K2 side:
https://help.k2.com/onlinehelp/k2blackpearl/icg/4.7/default.htm#Configure/Add_Multi_AD_Domains.htm
https://help.k2.com/onlinehelp/k2blackpearl/icg/4.7/default.htm#Prepare/SCC/MultiDomans.htm

2) Verify that all the proper connectivity is allowed between K2 server and this domain:
http://community.k2.com/t5/K2-blackpearl/Issues-getting-data-from-additional-domain/ta-p/94731
 

Resolution

There is only two places responsible in K2 which contain settings defining connection parameters for domains: Workspace management (User Managers > K2 > Domains) and Workspace web.config.
In this case settings in both places were seemingly in place and correct but after careful consideration it was noted that Netbios name for affected domain has not been specified correctly in Workspace User Managers settings causing error described above.