Thanks for your feedback. I took your advice and tried very version of JRE available.

In the case of Oracle JREs, they all appear to be linked to "oracle-java15-jre". See my attempt to install "oracle-java12-jre". There appears to have been some sort of license change and Oracle don't readily give access to earlier versions.

~$ sudo apt install oracle-java12-jre
Reading package lists... Done
Building dependency tree
Reading state information... Done
Note, selecting 'oracle-java15-installer' instead of 'oracle-java12-jre'
oracle-java15-installer is already the newest version (15.0.2-1~linuxuprising0).
The following packages were automatically installed and are no longer required:
linux-headers-5.4.0-62 linux-headers-5.4.0-62-generic
linux-image-5.4.0-62-generic linux-modules-5.4.0-62-generic
linux-modules-extra-5.4.0-62-generic python3-dbus.mainloop.pyqt5
python3-notify2
Use 'sudo apt autoremove' to remove them.
0 upgraded, 0 newly installed, 0 to remove and 2 not upgraded.


In the case of OpenJDK I have tried all the available versions.

~$ apt search openjdk| grep -e '-jre '
i openjdk-11-jre - OpenJDK Java runtime, using Hotspot JIT
i openjdk-13-jre - OpenJDK Java runtime, using Hotspot JIT
i openjdk-14-jre - OpenJDK Java runtime, using Hotspot JIT
i openjdk-8-jre - OpenJDK Java runtime, using Hotspot JIT

These equate to the following in LO.
Ubuntu 11.0.9.1
Private Build 13.0.4
Private Build 14.0.2
Private Build 1.8.0_275

Running the "Zotero_OpenOffice_Integration.oxt" file in each case results in me initially getting the information box:

-----------------------------------------------------------------------------------
You are about to install version 5.0.23 of the extension ‘Zotero
LibreOffice Integration’.
That version is already installed.
Click 'OK' to replace the installed extension.
Click ‘Cancel’ to stop the installation.
-----------------------------------------------------------------------------------

and when I do click connect I receive the following message:

-------------------------------------------------------------------------------------------------------
(com.sun.star.uno.RuntimeException) {{ Message = "JNI exception occurred”,
Context = (com.sun.star.uno.Xinterface) @7F1bcc0017F8
(ImplementationName = "com.sun.star.comp.stoc. JavaVirtualMachine") } }
-------------------------------------------------------------------------------------------------------

I have been using Zotero for over a decade now, it has gotten me through a MSc dissertation and a PhD thesis. It has worked pretty solidly over that time, except for a bit of a hickup when the standalone version was forced instead of the browser versions. I would really like to continue using it and I fear that I have done as much in terms of reporting as I can (I will also open a ticket with LO but you might have more clout to get something fixed, particularly if you can do some more detailed troubleshooting to point ot the exact problem.

I have submitted a bug to LibreOffice.

https://bugs.documentfoundation.org/show_bug.cgi?id=140015

Those aren't relevant. This has nothing to do with Zotero — it's entirely a problem in LibreOffice with your system Java configuration.

Again, see what I said above about trying a JDK from Oracle.

I'm saying to try the downloadable builds from Oracle, not anything that comes from your system package manager.

Have you tried a copy of LO downloaded straight from LO, instead of a packaged version?

You can also try deleting your LO user directory, which frequently fixes .oxt install problems, though usually with a different error.

Beyond that, you'll need someone from LO to help you.