I am having the same issue. In my case, a Raspberry Pi 2B was running Jessie. I wrote a PHP program accessing two solar charger controllers. It has worked since 2018.
The 2B's SD card crashed. I can not rebuild a card for Jessie, so now I am running Bookwork. The Modbus commands fail. I get the "Watchdog time expired [ 5 sec]!!! Connection to 192.168.1.1 is not established." error.
Any suggestions?