Two copies of the same plugin are installed
Applies to Paper, Vanilla.
What it looks like in your log
Ambiguous plugin name `Essentials' for files `plugins/Essentials.jar' and `plugins/EssentialsX.jar'
What causes it
Two jars in the plugins folder declare the same plugin name, usually an old version left behind next to a new one. Only one loads, and not necessarily the one you want.
How to fix it
Delete the older duplicate jar so only one file per plugin remains.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log