This issue started off with another mod but now points to Botania. Please bear with the intro.

I recently updated both my home server and client with BuildCraft 7.2.7 (from 7.2.6). I can start the server fine, however when my client connects the server forcefully closes and the client fails to connect. Both crash logs are linked below, as well as what the client error looks like when it fails to connect. I've tried reverting back to 7.2.6 but no luck.

BuildCraft Issue

As I should have, I took this up with the BuildCraft github tracker first. I got through the issue pointing towards BuildCraft, but now the crash logs indicate an issue with Botania which brings us to speed. Please refer to that tracker for specific steps I've taken to fix the BC issue.

Server 7.2.7 Fix1.1
Client 7.2.7 Fix1.1
Client Error

Buildcraft was updated with a handful of other mods. I reverted back to a previous, backed up world before the updates and began installing them one by one. I still don' tknow what caused the issue but it's fine now.