Set PluginClassLoader as parallel capable

The console line Set PluginClassLoader as parallel capable is an information message, not an error. It stands out because it is printed early in startup, before your plugins load, so it is easy to mistake for the cause when a server then fails. This guide explains what the line is and how to find what actually went wrong.

What the line means

It is logged once at INFO level by Spigot-based server software, while the server prepares the part that reads plugin jars. In published startup logs it shows up on Minecraft 1.8.8, 1.11.2 and 1.12.2 servers, always as a plain INFO line with no error beside it.

Current Paper does the same setup silently. Its plugin loader registers itself as parallel capable without writing anything to the console, so you may never see the line on a recent Minecraft version.

There is one related message that does point at a problem. The old Spigot code also has a WARNING line, Error setting PluginClassLoader as parallel capable, which is followed by a Java stack trace. Nothing on a current Paper server prints it.

If the server stopped after that line

1. Open the Console tab and scroll down from the line. Click Download Log and pick a range, such as Last hour, to search it on your computer.
2. Look for the first line that has Could not load ' or Error occurred while enabling after its timestamp. Either one names the plugin that failed. A line that contains Unknown/missing dependency plugins names a plugin you still need to install.
3. Open the Plugins tab, click Installed, and find that plugin. When the Installed list opens, the panel reads the log of the last start. A plugin the server could not load shows Failed to load, and one that needs another plugin shows Needs and its name.
4. Click Disable on the plugin, then Start the server from the Console tab. If it boots, the plugin was the cause.
5. Update the plugin or replace it with a build for your Minecraft version. Install plugins on Paper or Purpur covers installing and uploading by hand.

If the Installed list will not load, open the File Manager, go into the plugins folder and delete that one jar instead.

If the console is quiet after the line

If nothing useful appears below the line and the server still will not start, Find and read a Minecraft crash report and Fix mod and plugin errors cover the next steps.

Keep your version in step

A plugin that fails right after this line can be built for a different Minecraft version than the one your server runs. Check the plugin's own page for the versions it lists, then compare with the Minecraft version shown in the JAR Manager. Change your Minecraft version or mod loader shows how to move the server.