The exact same problem here! None of the proposed solutions seem to work. The following steps did _not_ work:

1. Deleting the extension folders in LibreOffice did not work:

`rd C:/Users/[user]/AppData/Roaming/libreoffice/4/cache/`
`rd C:/Users/[user]/AppData/Roaming/libreoffice/4/user/temp/`
`rd C:/Users/[user]/AppData/Roaming/libreoffice/4/user/extensions/`

2. Updating the Java JRE to the latest version dot not work (uninstalled all previous version, reinstalled a fresh, latest version).

3. Manual installation of the extension from `C:\Program Files\Zotero\integration\libreoffice` did not work (gave the error message).

Zotero: 7.0.4-beta.1+576e50552 (64-bit)
LibreOffice:
Version: 25.2.0.0.alpha0+ (X86_64) / LibreOffice Community
Build ID: 2d591ae8ac55195762e155f93c39c046a4bcba82
CPU threads: 24; OS: Windows 11 X86_64 (10.0 build 22631); UI render: Skia/Raster; VCL: win
Locale: en-CA (en_US); UI: en-GB

What is the version of JRE that is selected in LibreOffice Options -> Advanced dialog? What error are you getting when trying to manually install the extension?

I get the following error msg when double-clicking on Zotero_OpenOffice_Integration.oxt file to install it:

java.lang.ClassNotFoundException: org.zotero.integration.ooo.comp.RegistrationHandler

Thank you kindly for looking into the issue!

On my Windows 11, with Java 8 Update 421 (64-bit), the automatic installations fails. When I try to 'enable' the extension from within LibreOfficeDev, the following error message is emitted:

[jni_uno bridge error] UNO calling Java method writeRegistryInfo: non-UNO exception occurred: java.lang.UnsupportedClassVersionError: org/zotero/integration/ooo/comp/RegistrationHandler has been compiled by a more recent version of the Java Runtime (class file version 53.0), this version of the Java Runtime only recognizes class file versions up to 52.0
java stack trace:
java.lang.UnsupportedClassVersionError: org/zotero/integration/ooo/comp/RegistrationHandler has been compiled by a more recent version of the Java Runtime (class file version 53.0), this version of the Java Runtime only recognizes class file versions up to 52.0
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$100(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.net.FactoryURLClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at com.sun.star.comp.loader.RegistrationClassFinder.find(RegistrationClassFinder.java:53)
at com.sun.star.comp.loader.JavaLoader.writeRegistryInfo(JavaLoader.java:376)

When the error message is dismissed, another one pops up:

(com.sun.star.uno.RuntimeException) { { Message = "[jni_uno bridge error] UNO calling Java method writeRegistryInfo: non-UNO exception occurred: java.lang.UnsupportedClassVersionError: org/zotero/integration/ooo/comp/RegistrationHandler has been compiled by a more recent version of the Java Runtime (class file version 53.0), this version of the Java Runtime only recognizes class file versions up to 52.0\X000ajava stack trace:\X000ajava.lang.UnsupportedClassVersionError: org/zotero/integration/ooo/comp/RegistrationHandler has been compiled by a more recent version of the Java Runtime (class file version 53.0), this version of the Java Runtime only recognizes class file versions up to 52.0\X000d\X000a\X0009at java.lang.ClassLoader.defineClass1(Native Method)\X000d\X000a\X0009at java.lang.ClassLoader.defineClass(Unknown Source)\X000d\X000a\X0009at java.security.SecureClassLoader.defineClass(Unknown Source)\X000d\X000a\X0009at java.net.URLClassLoader.defineClass(Unknown Source)\X000d\X000a\X0009at java.net.URLClassLoader.access$100(Unknown Source)\X000d\X000a\X0009at java.net.URLClassLoader$1.run(Unknown Source)\X000d\X000a\X0009at java.net.URLClassLoader$1.run(Unknown Source)\X000d\X000a\X0009at java.security.AccessController.doPrivileged(Native Method)\X000d\X000a\X0009at java.net.URLClassLoader.findClass(Unknown Source)\X000d\X000a\X0009at java.lang.ClassLoader.loadClass(Unknown Source)\X000d\X000a\X0009at java.lang.ClassLoader.loadClass(Unknown Source)\X000d\X000a\X0009at java.net.FactoryURLClassLoader.loadClass(Unknown Source)\X000d\X000a\X0009at java.lang.ClassLoader.loadClass(Unknown Source)\X000d\X000a\X0009at com.sun.star.comp.loader.RegistrationClassFinder.find(RegistrationClassFinder.java:53)\X000d\X000a\X0009at com.sun.star.comp.loader.JavaLoader.writeRegistryInfo(JavaLoader.java:376)\X000d\X000a", Context = (com.sun.star.uno.XInterface) @0 } }

At the end, the extension remains disabled. Again, uninstalling the extension and reinstalling extension v. 5.0.23 fixes the issue.

The problem is that once Zotero is shut down and restarted, it automatically re-installs v. 7.0.1 of the LibreOffice extension over the working version, which, again, breaks the extension.

@EricBright: Try the latest beta.

Great, thanks very much to you and @jkr for reporting and helping us troubleshoot.