Now, this is what I did:
1. Uninstalled java.
2. Ran OpenJDK14U-jdk_x64_windows_hotspot_14.0.1_7.msi. This is practically the same what you did, only it installed java in c:\Program Files\AdoptOpenJDK\ (which I don't like and was not given the choice of choosing a different target). But at least I did not have to set the path variable manually, it did it for me.
3. "java -version" now prints exactly the same messages as yours.
And after this I installed Eubos 1.0.7, by selecting the jar file in Arena. And it still does not work.
Since then I deleted this installation and returned to the previous one.
EDIT: Here is an Arena debug when starting the engine.
2020-06-02 17:04:18,522-->1:quit
2020-06-02 17:04:18,897*1*--------------------Starting engine 1 Eubos 1.0.7 64-bit--------------------
2020-06-02 17:04:18,897*1*Configured Engine 1 Type: UCI
2020-06-02 17:04:18,897*1*Engine 1 dir: C:\Program Files\Java\jdk-14.0.1\bin
2020-06-02 17:04:18,897*1*Engine 1 commandline: "C:\Program Files\Java\jdk-14.0.1\bin\java.exe" -jar E:\Sakk\UCI-engines\Eubos\Eubos1.0.7.jar
2020-06-02 17:04:19,007*1*Child Process Prio Adj: PID 8164 conhost.exe
2020-06-02 17:04:19,007*1*Engine 1 ProcessID: 14700
2020-06-02 17:04:19,007*1*Engine 1 Prio:32 ThreadPrio:0
2020-06-02 17:04:19,038<--1:Exception in thread "main" java.lang.NoClassDefFoundError: com/fluxchess/jcpi/AbstractEngine
2020-06-02 17:04:19,038-->1:uci
2020-06-02 17:04:19,038<--1: at java.base/java.lang.ClassLoader.defineClass1(Native Method)
2020-06-02 17:04:19,038<--1: at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1017)
2020-06-02 17:04:19,038<--1: at java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:151)
2020-06-02 17:04:19,038<--1: at java.base/java.net.URLClassLoader.defineClass(URLClassLoader.java:514)
2020-06-02 17:04:19,038<--1: at java.base/java.net.URLClassLoader$1.run(URLClassLoader.java:422)
2020-06-02 17:04:19,038<--1: at java.base/java.net.URLClassLoader$1.run(URLClassLoader.java:416)
2020-06-02 17:04:19,038<--1: at java.base/java.security.AccessController.doPrivileged(AccessController.java:691)
2020-06-02 17:04:19,038<--1: at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:415)
2020-06-02 17:04:19,038<--1: at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:589)
2020-06-02 17:04:19,038<--1: at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
2020-06-02 17:04:19,038<--1: at java.base/java.lang.Class.forName0(Native Method)
2020-06-02 17:04:19,038<--1: at java.base/java.lang.Class.forName(Class.java:427)
2020-06-02 17:04:19,038<--1: at org.eclipse.jdt.internal.jarinjarloader.JarRsrcLoader.main(JarRsrcLoader.java:56)
2020-06-02 17:04:19,038<--1:Caused by: java.lang.ClassNotFoundException: com.fluxchess.jcpi.AbstractEngine
2020-06-02 17:04:19,038<--1: at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:435)
2020-06-02 17:04:19,038<--1: at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:589)
2020-06-02 17:04:19,038<--1: at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
2020-06-02 17:04:19,038<--1: ... 13 more
2020-06-02 17:04:27,059-->1:isready
That exception does not look good.