[Typo3] ERROR: File not written to disk! Write permission error in filesystem?

[Mathias Schreiber [wmdb>]]

Bert Bouwer wrote:
> I have upgraded from 3.7.0 to 3.8.0 , website is running fine,but, when 
> entering BE after login I get the following message:
> 
> ERROR: File not written to disk! Write permission error in filesystem?

TYPO3 tries to write files (locallang most likely).
For some reason it could not write those files.
Check permissions of the webserver user for the dir typo3conf and typo3temp

-- 
if ($GLOBALS['TSFE']->fe_user->user['ahnung'] == 0) {
	$this->fresseHalten = 1;
}