Hello HostHorde Community, I have been having this problem with a .jar file called Cauldron, Basicly Cauldron is a jar file that when started, you can use both Forge and Bukkit (plugins). There has been other threads about this, and when I came to solutions yesterday from HostHorde staff, it didn't help. Links are down below Link #1 Link #2
Joseph linked you to the knowledge base article that tells you how to fix the problem. You are either missing the library files (which can be downloaded using the Cauldron installer or here), or a correctly named Vanilla server jar.
There is no Cauldron installer on there. Sidenote, the knowledge base article does not seem to work. I have tried numerous times, as well as trying different things in the article. Thanks for your reply though
The installer and server files were here, but right now they're unavailable because Cauldron was also DMCA'd along side Bukkit, Spigot, etc. There are working downloads links for 1.5.x and 1.6.x in the EAQ though. The forge installers might also be able to provide you with what you need: http://files.minecraftforge.net/
Sorry it took me long (yet again) and thank you! Can't believe it was DMCA'd though, but yet again, thank you!