Set permission on ListItem level. Issue with usernames with Norwegian letters

I have created an EventReceiver that is setting permissions on ListItem level. I am having some problems with users having norwegian letters (æøå) in their username. For other users it works.

Under you will find a part of the code setting permissions. The EventReceiver is running with RunWithElevatedPrivileges.

if (properties.ListItem["Responsible_x0020_for"] != null)
                                            {
                                                //Removed Responsible_x0020_for_x0020_acti added Responsible_x0020_for
                                                SPRoleAssignment roleAssignmentResponsibleAction = new SPRoleAssignment((SPPrincipal)web.AllUsers[GetUserFromAfterProperties(properties.ListItem["Responsible_x0020_for"], web).LoginName.ToString()]);
                                                roleAssignmentResponsibleAction.RoleDefinitionBindings.Add(web.RoleDefinitions.GetByType(SPRoleType.Contributor));
                                                reporting.RoleAssignments.Add(roleAssignmentResponsibleAction);

                                            }
                                            else if (getSPUserValue(properties).LoginName != null)
                                            {

                                                SPRoleAssignment roleAssignmentResponsibleAction = new SPRoleAssignment((SPPrincipal)web.AllUsers[getSPUserValue(properties).LoginName.ToString()]);
                                                roleAssignmentResponsibleAction.RoleDefinitionBindings.Add(web.RoleDefinitions.GetByType(SPRoleType.Contributor));
                                                reporting.RoleAssignments.Add(roleAssignmentResponsibleAction);

                                            }

The strange thing is that it works if the user is in a group, but not if it is a single user field. Maybe it is that the group is handled as the object vs the single user is handled as the object?

I have tested with several users containing æøå in their usernames.

Any help appreciated