TYPO3 Exception thrown but not shown in FE/BE - typo3

Question

How can I show Exception in Debug Mode (Install -> Presets) in FE?
I'm using LTS 7.6.29
My TYPO3 configuration is
1 BE/debug
1 FE/debug
* SYS/devIPmask
1 SYS/displayErrors
file SYS/enableDeprecationLog
1 SYS/sqlDebug
0 SYS/systemLogLevel
1 SYS/clearCacheSystem
config.contentObjectExceptionHandler = 0
PHP.ini
error_reporting = E_ALL
display_errors = On
display_startup_errors = On
log_errors = On

Answer 1

You've to use this setting in TypoScript to show Error-Messages in Frontend:
config {
contentObjectExceptionHandler = 0
}
In the file LocalConfiguration.php you've to set following values, it's explained how to set them by using the installtool:
SYS
Try, to disable the TYPO3 exception handlers in general:
System > Install > "All Configuration"
Filter by (search-field): exceptionhandler
Set empty values:
[SYS][productionExceptionHandler] =
[SYS][debugExceptionHandler] =
[SYS][errorHandler] =
After debugging set exception handling to the default values:
[SYS][productionExceptionHandler] =
TYPO3\CMS\Core\Error\ProductionExceptionHandler
[SYS][debugExceptionHandler] = TYPO3\CMS\Core\Error\DebugExceptionHandler
[SYS][errorHandler] = TYPO3\CMS\Core\Error\ErrorHandler
After changing the settings you've to delete the file typo3temp/autoload/autoload_classmap.php that the changed settings are applied to the system. The file will be new created but with a bit different content.
You can delete the file too in installtool by deleting the autoload-information.
Most is explained here: https://docs.typo3.org/typo3cms/extensions/launch/Administrators/03_Errorhandling/09_OopsAnErrorOccured/Index.html