Hello Jan:
It looks like some of the parameters in your brain input box may be incorrect.
I don't have access to my machine at this time. Look at the brain videos Art has done, and write down some notes.
The different boxes relating to the correct format in the brain are not really clear or intuitive.
The wording in the "Device" column is for information only, the numbers appearing in that label have no bearing.
Also, in the modbus test box, you have "Format" Hex , and a decimal value in the test box area. Just a guess here. However, you did get an input showing in the test box, so the modbus info is getting read into Mach. It is the brain that outputs the data into the program, so you are halfway there.
I am no expert in brains, but remember when setting up my modbus, it took a lot of fiddling and trying different numbers in the brain entry boxes before I got it working correctly.
It is not an issue with a bug in Mach3. It is just one of those tricky things that need working out

One little wrong number in the wrong box, and nothing works!!
Sorry to not have more helpful information. I am away (in another state this next two weeks) from my machine, otherwise I could do a point by point set-up for a brain input from my own working system. I have (3) potentiometers, 13 pushbuttons, and a keypad all going through my modbus. Once it is working, one tends to forget about it. It just grinds away in the background, like a good liitle slave.

If you work with just one modbus input, and one brain until it works, then you can move to the next. You may have modbus interacting if the numbers are not right. They are using the same address. Just turn the enable off on one.
I do remember, the potentiometers all had increasing address's.