dont know if youve been told this already, but make sure youve done all these:
1. make sure youre using Wifi on your Wii u and not a LAN cable. Sometimes it works, sometimes it doesnt. Maybe i was doing something wrong but just use wifi to be safe
2. Try disabling your antivirus, sometimes they interfere.
3. make sure you have the correct Wii U IP address, you can get this from the TCPgecko installer, right when you're about to press A to install, it displays your wii u's ip on the top.
Make sure Auto Connect is unchecked in Jgecko, and place that in the box beside the checkbox.
4. make sure you're using the same wifi network as your wii u. if you have two like i do, sometimes your computer may connect to the other one randomly and it causes connection issues.
Any error that pops up in Jgecko has nothing to do with any of my codes. Unless they crashed the game somehow, but thats unlikely.
Beyond these steps i cant think of what else could be wrong.