Common issues

Local-hosting (e. g. resource packs)

A lot of resource pack plugins allow local-hosting, which is fine by itself, as long as they bind to a different port than the one the minecraft server stands on. That should usually be the case, however, because many hostings firewall other ports by default, some plugins resort to using the same port as the mc server, and using a custom handler to redirect the traffic to serve an http response. This is (usually) not possible to do safely tho, as Alix stands at the top of the layer 7 networking foodchain, and will intercept the packets before they reach the main server. An http request will look like gibberish to it, and, trying to parse it as a minecraft packet, will result in Alix firewalling the player.

TL;DR: Do not let anything else bind to the same port as the minecraft server, like a locally-hosted resource pack service

Solution: Force the plugin to use a different port or contact the plugin developers, and tell them that you have a security layer that parses minecraft packets, and that they need to bind to a different port