How to fix this problem

[Zenon-Ultra]

trafficstars

[Codixer]

You placed a config file (betterspawnerconfig.json) in the optionalon mods. Place those in the files instead, so D:\TestRoot2\servers\qda_-1.16.5\files\<all your config files/folders go here>.

files - Any files like configs, resourcepacks or other stuff you want to send and validate to the client optional(on/off) - Mods that people don't need to join your server/launch the game. Depending on the on/off, it'll be off or on by default for people to choose if they want to enable or disable a mod. required - These mods will always be downloaded to the client, Helios (or your fork) will always download these mods.

Or if you like to have nebula as a (commercial) service. Look into https://github.com/dscalzi/HeliosLauncher/discussions/335