TYPO3 Core

Running on Php 5.x, this error appears sometimes (I cannot reproduce) and then dies.

Fatal error: Cannot use string offset as an array in XXXX/tslib/class.tslib_fe.php on line 1328

It occurs majorly after another any other php error (not related) happened.

This is the line:

if ( $this->TYPO3_CONF_VARS['FE']['debug'] || $this->config['config']['debug'] ) {
$this->content.=chr(10).'';
}

The only way to mitigate I found is comment the whole if block.
(issue imported from #M6666)