2819735 - 500 Internal Server Error when creating SOLMAN_BTC user in step 'Maintain Technical Users' in System Preparation

In step 'Maintain Technical Users' in System Preparation, you create the user SOLMAN_BTC, however 500 Internal Server Error appears when clicking the button 'Execute'. When you go to ST22, the following dump can be also seen,

Category               ABAP Programming Error
Runtime Errors         CONVT_NO_NUMBER
Except.                CX_SY_CONVERSION_NO_NUMBER
ABAP Program           CL_SISE_ACT_CUSTOM_AGS========CP
Application Component  SV-SMG-INS-CFG

Short Text                                                                                        
    "OS_RFC" cannot be interpreted as a number

Error analysis                                                                                    
    An exception has occurred which is explained in more detail below. The                        
    exception is assigned to class 'CX_SY_CONVERSION_NO_NUMBER' and was not caught                
     in procedure                                                                                 
    "SAVE_AISUSER" "(METHOD)", nor was it propagated by a RAISING clause.                         
    Since the caller of the procedure could not have anticipated this                             
    exception, the current program was terminated.                                                
    The reason for the exception is:                                                              
    An attempt was made to interpret value "OS_RFC" as a number.                                  
    As this value contravenes the rules for displaying numbers correctly,                         
    this was not possible.

|   41|    IF at_list_aisuser IS INITIAL.                                                          |
|   42|      REPLACE FIRST OCCURRENCE of 'S' in lv_s_username WITH ''.                             |
|>>>>>|      lv_i = lv_s_username.                                                                 |
|   44|      lv_s_username = '' + lv_i .                                                           |
|   45|      SELECT solman_user FROM aisuser INTO TABLE at_list_aisuser WHERE parnr = lv_s_username|
|   46|    ENDIF.

SAP Solution Manager 7.2

500 Internal Server Error; SOLMAN_BTC;"OS_RFC"; , KBA , SV-SMG-INS-CFG , Setup and Configuration of the Solution Manager system , Problem