[Typo3] Problems installing extensions

[David Cake]

	I have a typo3 install (3.8.0 upgraded from 3.6, running on 
Debian Linux) in which extensions do not seem to install.
	If I click on install, it tells me the database needs to be 
updated (or other install as necessary), and then when I click 
through, I am returned to the Ext manager screen showing the 
extension as uninstalled. No error message, just installing doesn't 
seem to do anything.
	I am fairly sure that its not a mysql permissions problem - 
the problem seems to persist even if mysql runs as the root user who 
should have permission to make any changes.
	Any ideas where I start looking to debug this?
	Cheers
		David