Description:
I am using Ubuntu 16.04.4
I did a fresh installation of LO 6.0.2.1 from Ubuntu ppa repo.
I can't install some extension. I tried:
LibreWeb 1.0.7 
Error:
"Failed to register package for vnd.sun.star.expand:$UNO_USER_PACKAGES_CACHE/uno_packages/lu26543jw5oy7.tmp_/LibreWeb-user-1.0.7.oxt/LibreWeb/"

LanguageTool 4.0
Error:
"[jni_uno bridge error] UNO calling Java method writeRegistryInfo: non-UNO exception occurred: java.lang.NoClassDefFoundError: com/sun/star/task/XJobExecutor
java stack trace:
java.lang.NoClassDefFoundError: com/sun/star/task/XJobExecutor
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:763)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:467)
	at java.net.URLClassLoader.access$100(URLClassLoader.java:73)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:368)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:362)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:361)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:411)
	at java.net.FactoryURLClassLoader.loadClass(URLClassLoader.java:814)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	at com.sun.star.comp.loader.RegistrationClassFinder.find(RegistrationClassFinder.java:53)
	at com.sun.star.comp.loader.JavaLoader.writeRegistryInfo(JavaLoader.java:376)
Caused by: java.lang.ClassNotFoundException: com.sun.star.task.XJobExecutor
	at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	... 15 more"

Writer2epub 1.1.28 worked.
So I went back to a fresh installation of LO 5.4.5.1
I tried to install the extensions (the installation took 1 moment) but doesn't work. I tried to uninstall LT, but return me this error:
"(com.sun.star.lang.IllegalArgumentException) { { { Message = "Cannot detect media-type: file:///home/domenico/.config/libreoffice/4/user/uno_packages/cache/uno_packages/lu5950ir81ei.tmp_/LanguageTool-4.0.oxt", Context = (com.sun.star.uno.XInterface) @4f98620 } }, ArgumentPosition = (short) -1 }"

What can I do?

Actual Results:  
Extensions not working, error during uninstall:
"(com.sun.star.lang.IllegalArgumentException) { { { Message = "Cannot detect media-type: file:///home/domenico/.config/libreoffice/4/user/uno_packages/cache/uno_packages/lu5950ir81ei.tmp_/LanguageTool-4.0.oxt", Context = (com.sun.star.uno.XInterface) @4f98620 } }, ArgumentPosition = (short) -1 }"

Expected Results:
Extension working :)


Reproducible: Didn't try


User Profile Reset: Yes



Additional Info:


User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:58.0) Gecko/20100101 Firefox/58.0