I have a Minecraft server at Nitrado, but it keeps crashing because of a lot of mods and says that it has too little RAM. Actually the server has 6GB RAM, unfortunately it only uses a maximum of 3 GB.
I once had a similar problem. For me the CPU was overloaded
It was a problem with Nitrado. They didn't configure Java correctly, that could only use 3 GB. It's cheeky.