When CreativeCore_NEOFORGE_v2.14.1_mc1.21.8 , AmbientSounds_NEOFORGE_v6.1.10_mc1.21.8 are installed
are installed , the client refuses to connect to a Paper Server, Maybe even spigot. This is a new bug.
Did you forget to compile with those API's included? Removing these mods restores client functionality.

New version reverts back to old behavior. Sorry for the trouble and my response was not really correct. I overlooked that I have no reverted entirely to the previous state and assumed it is an issue of NeoForge.

Problem is that NeoForge does not led you connect because the server is missing CreativeCore. I do not see a way to fix this from my end, unfortunately.

You may want to State on the download page that AmbientSounds will no longer work in your client on
Java Servers such as Spigot or Paper , ETC ...