Okay, this may work or not. (It worked for me when I hosted dedicated servers.) Okay, first start up Blockland normally. Then hit Start Game, and hit Add-ons, then enable disable the ones you want enabled for the dedicated server. Close down the Add-ons, and start your non-dedicated server. After everything is loaded and your ingame, go ahead and leave.
Then start-up your dedicated server, and see if it works.