There is a bug in the 3.0.42 client of Unstable 1.8.
After connecting to the server the Just Enough Items (JEI) mod on the client machine is trying to communicate with Just Enough Items mod on the server. However, the version of JEI in the modpack is flagged as a clientside only mod and is automatically disabled on the server. When the server receives the packet meant for JEI it cannot handle it and this crashes the connection. The client eventually times out and disconnects.
How to fix this
You will have to disable the Just Enough Items mod and the mods dependant on it on your client:
In FTB Launcher:
- Select the Unstable 1.8 modpack
- Click the Edit Mod Pack' button
- For each of the following entries, click to highlight them and click the 'Disable' button:
You should now be able to launch the modpack and connect to your server.