Ok, here are some tips to avoid problems with running Forge servers.
This currently applies for version 1.15.2, build 31.1.17.
I strongly recommend that the connection be direct, that is, without a proxy in between. To date, BungeeCord does not offer support for Forge, so it is better not to waste time trying. If you decide to use VelocityProxy (which does offer support for Forge servers), I recommend using the modern player info forwading. Why? Because if you use "legacy" or, failing that, none, you will get immediate disconnection errors due to the inability to find a valid uuid. You may also get errors such as that you must install the Forge client when you already have it, or that you need certain mods when you actually have them all.
Currently it is not possible to connect through VelocityProxy with the modern system, since «Fabric» (with which it is possible to link the Forge server with the proxy) does not offer support for the latest build, and it is for this reason that I advise to use a Direct connection to the Forge server.
I remind CentOS 7 users (server side) that they must configure the iptables and the firewall correctly, to avoid errors of the type "java.net.NoRouteToHostException: No route to host", among others.
When it comes to installing mods, I recommend installing them all one by one. In case one has an error, they will know what it is.
Something important to remember is that, if you are going to install the mods one by one, you must delete the “world” folder before including a new mod.