Hello. I am not sure if this is the right place to put this thread, so feel free to correct me if I'm in the wrong. On my server (Spigot 1.11), players (including myself, the OP), cannot put armor on armor stands. They can be placed/broken. It doesn't matter if it's in an area claimed by GriefPrevention/WorldGuard, or in the wild. I'm honestly not sure what could be causing this, as it works just fine in singleplayer and on other servers.
My plugins list should be in an attachment to this post. If any more information is needed, I will gladly supply it, I'm just not sure what's causing this. I haven't found anything else about armor stands not working...
No plugins on the list are red, but are there any errors in Console? You really don't need Towny + GriefPrevention...try removing of of those. If that doesn't help then you'll have to remove one-by-one and restart each time. Though, it might be easier to copy everything from the plugins to your computer and use LANBroadcaster to create a test server (you will not need the world files from the server), but in this case add them one-by-one until the armor stand problem starts (this will work much faster and will not inconvenience your players).
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For immediate assistance, I can usually be found in-game at: play.haleymc.net
When I've checked, the only errors in the console that I've seen are regarding the "unregistered PluginClassLoader". Removing GriefPrevention doesn't fix it. When I get time I'll go through each plugin and report my results here.