The ModbusTCP server and client are started at boot time before the RevPi has initialized its Ethernet port and cannot open their sockets. The problem could only be fixed, if a reset was made with
piTest -x later.
This problem is fixed in the ModbusTCP server and client now.
You can get the new version for Jessie with
sudo apt-get update
sudo apt-get upgrade
If you are still working with Wheezy and need the update, please send me a message.