Aromat touch-screens and MODBUS

I wish to use an Aromat GT-01 touch panel HMI in MODBUS RTU mode over RS-232. I've written my own slave code and have tried a third party
slave tester with this panel. Both work fine with a MODBUS master-simulator and have been checked against the MODBUS written specification.
The display probes the slave with function 0x10, and the slave returns a successful-write response, and the display puts ER01 in the upper right corner. The cycle repeats forever (normal polling perhaps) but the error code sticks and my widgets never appear. I'm running the latest firmware. A GT-01 with earlier firmware displays ERFE. Neither error code is documented in the latest techical manual.
I've had trouble with bugs in the Aromat firmware when using their general purpose ASCII protocol (the response from them was, essentially, "too bad"), so I'm leery of their MODBUS RTU implementation. Is anyone successfully using an Aromat GT-series touch panel with MODBUS?
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