Server Software and Version:
13:11:01 [INFO] Linux 4.15.0-122-generic [x64],
13:11:01 [INFO] Java 1.8.0_272 [x64],
13:11:01 [INFO] BungeeCord git:BungeeCord-Bootstrap:1.16-R0.4-SNAPSHOT:9c078b7:1539,

Hello,

first of all, please refer to this issue: ME1312/SubServers-2#56 (comment)

So basically, this plugin somehow does not work with SubServer, but I cannot find some easy reason why.

It works fine when used with bungeecord itself, but it only shows "Cannot connect to the server" when the bungeecord is running through SubServers.

Any idea?

I wouldn't exactly call AdapterComponent standard API.

Anyway, after further investigation, the problem seems to be that your plugin is submitting a blank description object to BungeeCord when running alongside SubServers.

ping() placed at the end of the event queue
done() placed at the equivalent to here in BungeeCord's code

Also, it seems to be running a duplicate event, which goes unused.

when running alongside SubServers

The implementation of the description component is irrelevant. MiniMOTD emits proper json and sets the description using the standard bungeecord api, as mentioned before. It does not emit a blank description object under normal circumstances/without SubServers (at least, I can't reproduce this).
Not sure what you are on about with this duplicate event either, the listener is only registered once, so I can only assume the duplicate event is due to something in your testing environment.

In my opinion the most likely suspects in order here are

1. SubServers
2. Bungeecord
3. The adventure bungeecord platform adapter

If this issue is reproducible without SubServers (or if more useful information pointing to how MiniMOTD is causing this is discovered) I will look into it further. In that case, please open a new issue. If you have any other questions you can also join my Discord.

I set the MOTD using the standard API, ServerPing#setDescriptionComponent, nothing out of the ordinary. I have no idea what is causing your issue and I do not know of or use SubServer. If MiniMOTD is working without SubServer but not with SubServer, I can only assume SubServer is causing the problem. I test MiniMOTD with Waterfall, not BungeeCord, so it may be worth giving that a try, however I don't see how it would make a difference to be honest.