I am trying to make a story line in my server. I started making a quest and it worked like a glove. But then I tried to make a second quest, and it didn't work. After I tried to add a "Stage" (Deliver Items) and pressed "Done" nothing happend. It just doesn't do anything anymore, so I can't even type in the chat or reload the plugin. Here the Warning: https://pastebin.com/35B0mJPz.

Please help me so I can move on with this awesome plugin :)

The plugin needs to be properly updated to 1.13 and the changed Vault API (the items library was removed from the VaultAPI in the 1.13 update as it's not really necessary anymore)

A bunch of servers I work with have the same issue.
https://pastebin.com/03Hfv78N

This isn't vault related. Vault doesn't provide any quests or questing interfaces.

Is @Sleaker the author of this plugin?

It is vault related. The error is a missing class in Vault: java.lang.NoClassDefFoundError: net/milkbowl/vault/item/Items
This isn't an error about questing.

That does look vault related... Is there any way I can help with this issue? (I actually can't figure out how to use the vault API, so I might not be able to do much)

Is there any way I can help with this issue?

Contact the author of the plugin and request that he updates it?

If @Phoenix616 is correct with the initial statement, then I'm starting to think that @Sleaker is actually correct. Quests needs to not try and use the Item API in Vault since it isn't needed. @Phoenix616 , can you confirm?

So how do I depend on vault and use the API? When I tried, I got an error (Does not create jar using ANT, or something like that). Could you at least send me a link to a good article on how to do this or fix the error (if it is a problem with the plugin).

All, the reason why this is not Vault related is because the Item API no longer exists. It was removed in the last release due to material updates. Plugins that were previously using this API need to update and use the new Material API provided by Bukkit. The Item API hasn't worked as well as it needed to for some time now due to item tags.