Forwarding VS - with selective irule for SRC to DST only allowed to access (using data groups) - syntax fail

it seems okay here. you have created data groups, haven't you?

root@(ve11c)(cfg-sync In Sync)(Active)(/Common)(tmos) list ltm rule qux
ltm rule qux {
    when CLIENT_ACCEPTED {
  if { ([class match [IP::client_addr] equals L2L_SITEA_SUBNETS] && [class match [IP::local_addr] equals L2L_SITEB_SUBNETS]) } {
    forward
    return
  } elseif { ([class match [IP::client_addr] equals L2L_SITEB_SUBNETS] && [class match [IP::local_addr] equals L2L_SITEA_SUBNETS]) } {
    forward
    return
  } else {
    drop
  }
}
}

Yes Data Groups have been defined (each contains multiple subnets). Removed the word "return" for the if and the elseif statement - which fixed issue