[Typo3] Fatal error after upgrade to 3.8.1

[Nige]

Hi,

I just started to upgrade my site to 3.8.1 (from 3.7.0).

I only got to step 1...copied /media, /typo3 etc etc...when I tried to 
log into the install tool to compare the db and update static data I got 
this error...

TYPO3 Fatal Error: Extension key "sv" was NOT loaded! 
(t3lib_extMgm::extPath)

...got the same in the FE, and also trying to get into the BE.

I can see the sv extension in typo3/sysext, but what next?...

Nige