java.io.IOException: Can't process class [class path] (Unsupported version number [55.0] (maximum 54.0, Java 10))

I'm unable to compile java code that my coworker can successfully compile.

I have JRE 8u221.

Proguard is version 6.03

From my build.bat output:

dist-obfuscate: [echo] Extracting Objfuscator [unzip] Expanding: c:\Workspaces\abcd\efg\Trunk\ESDE\Scripts\proguard\proguard6.0.3.zip into c:\Workspaces\abcd\efg\Trunk\ESDE\Scripts\proguard [echo] Jarring Base ESDE [copy] Copying 1 file to c:\Workspaces\abcd\efg\Trunk\ESDE\Scripts\proguard

 [java] ProGuard, version 6.0.3java.io.IOException: Can't read [c:\Workspaces\abcd\efg\Trunk\ESDE\Scripts\proguard\sdeIn.jar] (Can't process class [company/application/actions/AboutAction.class] (Unsupported version number [55.0] (maximum 54.0, Java 10)))

 [java]
 [java] Reading input...
 [java]     at proguard.InputReader.readInput(InputReader.java:214)Reading program jar [c:\Workspaces\abcd\efg\Trunk\ESDE\Scripts\proguard\sdeIn.jar]
 [java]
 [java]     at proguard.InputReader.readInput(InputReader.java:184)
 [java]     at proguard.InputReader.readInput(InputReader.java:162)
 [java]     at proguard.InputReader.execute(InputReader.java:90)
 [java]     at proguard.ProGuard.readInput(ProGuard.java:255)
 [java]     at proguard.ProGuard.execute(ProGuard.java:96)
 [java]     at proguard.ProGuard.main(ProGuard.java:572)
 [java] Caused by: java.io.IOException: Can't process class [company/application/actions/AboutAction.class] (Unsupported version number [55.0] (maximum 54.0, Java 10))
 [java]     at proguard.io.ClassReader.read(ClassReader.java:114)
 [java]     at proguard.io.FilteredDataEntryReader.read(FilteredDataEntryReader.java:87)
 [java]     at proguard.io.FilteredDataEntryReader.read(FilteredDataEntryReader.java:87)
 [java]     at proguard.io.JarReader.read(JarReader.java:83)
 [java]     at proguard.io.DirectoryPump.readFiles(DirectoryPump.java:65)
 [java]     at proguard.io.DirectoryPump.pumpDataEntries(DirectoryPump.java:53)
 [java]     at proguard.InputReader.readInput(InputReader.java:210)
 [java]     ... 6 more
 [java] Caused by: java.lang.UnsupportedOperationException: Unsupported version number [55.0] (maximum 54.0, Java 10)
 [java]     at proguard.classfile.util.ClassUtil.checkVersionNumbers(ClassUtil.java:148)
 [java]     at proguard.classfile.io.ProgramClassReader.visitProgramClass(ProgramClassReader.java:101)
 [java]     at proguard.classfile.ProgramClass.accept(ProgramClass.java:430)
 [java]     at proguard.io.ClassReader.read(ClassReader.java:91)
 [java]     ... 12 more
 [java] Java Result: 1

BUILD FAILED c:\Workspaces\abcd\efg\Trunk\ESDE\build.xml:877: Warning: Could not find file c:\Workspaces\abcd\efg\Trunk\ESDE\Scripts\proguard\sde.jar to copy.

Questions:

1) "Version 55.0"? Version of...what?

Proguard version = 6.0.3.

2) "Java 10"? Nope.

java -version java version "1.8.0_221" Java(TM) SE Runtime Environment (build 1.8.0_221-b11) Java HotSpot(TM) 64-Bit Server VM (build 25.221-b11, mixed mode)

This is Java 8, not 10.