TYPO3 Core

Wow - 3.7.0. Did you consider upgrading to 4.x recently? ;-)

Beginning on line 1205 in class.tslib_pibase.php there's the following:
function pi_getFFvalue($T3FlexForm_array,$fieldName,$sheet='sDEF',$lang='lDEF',$value='vDEF') {
$sheetArray = $T3FlexForm_array['data'][$sheet][$lang];
if (is_array($sheetArray)) {
return $this->pi_getFFvalueFromSheetArray($sheetArray,explode('/',$fieldName),$value);
}
}

Thus, I guess there is somewhere a misconfigures XML/flexform or a wrong disposal of pi_getFFvalue() in one of your extensions. Did you possible upgrade a extension the last few days?

You can user the PHP function debug_print_backtrace() at the position the error was thrown (line 1206) to get some more information about which extension/part is wrong...

Won't fix unless you prove that the bug still exists in 4.1.x