Hey there,
If you have spawned in a lot of entities (mobs, items, etc.) and can't join your server, follow these steps.
1. Start your server.
2. Do NOT join your server when you start it. Instead, through the server console, run the command "/minecraft:kill @e[type=(entity)]". Replace "(entity)" with the entity you spawned in. In this case, it would be "item".
3. Join your server!