LM as ModbusSlave. Transaction ID error

Code:
if not mb then
  require('genohm-scada.eibdgm')
  require('luamodbus')

  -- list of coil mapping, starting from 0
  coils = {}

  -- list of register mapping, starting from 0
  registers = {
    {'40/1/5',dt.float32,0},{'40/1/6',0xFF,1}
 }
  holdingRegisters = {
    {'40/1/7',dt.uint16}
 }
  
  -- list of register data types, element count must match registers table
   
    -- knx group write callback
  function knxgroupwrite(event)
    local value

    -- try to find matching coil
    for id, addr in ipairs(coils) do
      if event.dst == addr then
        value = knxdatatype.decode(event.datahex, dt.bool)
        mb:setcoils(id - 1, value)
      end
    end
    
    -- try to find matching holding register
    for id, addr in ipairs(holdingRegisters) do
      if event.dst == addr[1] then
        value = knxdatatype.decode(event.datahex,addr[2])
        mb:setregisters(id-1, value)
      end
      end

    -- try to find matching input register
    for id, addr in ipairs(registers) do
      if event.dst == addr[1] then
        if (addr[2]==dt.float32) then
          value = knxdatatype.decode(string.sub(event.datahex,1,4), dt.uint32)
        mb:setinputregisters(1+addr[3], value)
            value = knxdatatype.decode(string.sub(event.datahex,-4), dt.uint32)
      mb:setinputregisters(addr[3], value)
            elseif (addr[2]==0xFF) then
            else 
              value = knxdatatype.decode(event.datahex,addr[2])
              mb:setinputregisters(addr[3], value)
            end
      end
    end
  end

  -- coil write callback
  function mbwritecoils(coil, value)
    local addr = coils[ coil + 1 ]
    if addr then
      grp.write(addr, value, dt.bool)
    end
  end

  -- register write callback
function mbwriteregisters(register, value)
local addr = holdingRegisters[ register + 1 ][1]
    if addr then
  grp.write(addr, value, holdingRegisters[ register + 1][2])
    end
  end

  -- knx group monitor, handles group writes
  knxclient = eibdgm:new({ timeout = 0.1 })
  knxclient:sethandler('groupwrite', knxgroupwrite)

  -- modbus slave, listen on all interfaces and default port 502
  mb = luamodbus.tcp()
  mb:open('0.0.0.0', 502)

   --setting slave id is optional
   --mb:setslave(1)

  mb:setreceivetimeout(0.1)
  mb:setmapping(#coils, 0, #holdingRegisters, #registers)

  -- init coils
  for id, addr in ipairs(coils) do
    value = grp.getvalue(addr)
    mb:setcoils(id - 1, value)
  end

  -- init holding registers
  for id, addr in ipairs(holdingRegisters) do
    value = grp.getvalue(addr[1])
    mb:setregisters(id - 1, value)
  end
  
-- init registers
  for id, addr in ipairs(registers) do
    if (addr[2]==dt.float32) then
    elseif(addr[2]==0xFF) then
    else
      value = grp.getvalue(addr[1])
      mb:setregisters(addr[3], value)
  end
  end
  
  -- set callbacks for coil and register write
  mb:setwritecoilcb(mbwritecoils)
  mb:setwriteregistercb(mbwriteregisters)
end

-- handle modbus and knx
mb:handleslave()
knxclient:step()