Morning,
This is not usual to have some many plcs, particulary small ones likes TSX37 and hmis connected together in Modbus.
Are you sure by the way that it's Modbus and not Unitelway wich is the Telemecanique protocol usually used to connect Schneider Hmi to plc ?
Anyway, Modbus or Unitelway it sound like "wires troubles". You have to check all the cables and connections first. Then you must verify that there's no conflict of adresses in the hmis and plcs. Some views of the configurations could be usefull to go further.
You said you have changed an hmi but how, as it seems you don't have the version needed to open old projects ?
JC