Java library only working on java 1.8

For a university assessment we're doing a large java project which makes use of a given external hardware that utilises a given java library to interact with it.

The main problem is that such java library only runs on java 1.8 (the reason is not clear yet) and just fails with newer versions of java.

Since the rest of the code (a few thousands) is written in java 9, it's obviously a hard task to rewrite everything without making uses of all the functionalities added since java 1.8

I have the following questions:

• Is there a way to make the whole project back-compatible without changing thousands of lines of code? (or to make the library work with a newer version of java)
• In the case there isn't such way, is there an easy way to see what needs to be changed to make the project compatible with a previous version of java?

Thank you in advance for any answer, any small contribution will be a great deal for us

Error stack trace:

# A fatal error has been detected by the Java Runtime Environment:
#
#  EXCEPTION_ACCESS_VIOLATION (0xc0000005) at pc=0x0000000180005b00, pid=20224, tid=10952
#
# JRE version: Java(TM) SE Runtime Environment (9.0+11) (build 9.0.4+11)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (9.0.4+11, mixed mode, tiered, compressed oops, g1 gc, windows-amd64)
# Problematic frame:
# C  [rxtxSerial.dll+0x5b00]
#
# No core dump will be written. Minidumps are not enabled by default on client versions of Windows
#
# An error report file with more information is saved as:
# C:\Users\works\Documents\GitHub\Software-Engineering\src\hs_err_pid20224.log
#
# If you would like to submit a bug report, please visit:
#   http://bugreport.java.com/bugreport/crash.jsp
# The crash happened outside the Java Virtual Machine in native code.
# See problematic frame for where to report the bug.