His 1st way is ok for me, i can refresh without error.
The solution on stackoverflow doesn't work for now...

I think you need to remove this line from app*.php:
https://github.com/symfony/symfony-standard/blob/3.3/web/app.php#L7

Try also removing L12 btw (loadClassCache)

@nicolas-grekas That's indeed the second solution he found. However, I'm wondering why it happens and if it's a good idea to release standard code for Symfony that provoke this issue. I don't know the caching mechanism in details, but I guess there's an impact on performance for both solutions (disabling the inline dumper or the loadCacheClass) ?

I was also thinking that these code lines in app(_dev).php are not at all documented, and it might make sense to comment them (especially if they can or should be removed under certain circumstances). WDYT ?

Alternatively, can you try to turn container.dumper.inline_class_loader to false? and then to remove it?
(while reverting the previous change)

So are those 2 settings (loadClassCache and inline_class_loader) incompat? Is there something that needs fixing or not?

There are incompatible, but that should be already handled:
https://github.com/symfony/symfony/blob/3.4/src/Symfony/Component/HttpKernel/Kernel.php#L842

dunno what's missing - maybe a PHP version check also, adding \PHP_VERSION_ID >= 70000 to the existing check.

@gnutix some more debugging hints would help (also trying this additional check please)

Setting container.dumper.inline_class_loader to false resolves the issue. Same when using:

'inline_class_loader_parameter' => \PHP_VERSION_ID >= 70000 && !$this->loadClassCache && !class_exists(ClassCollectionLoader::class, false) ? 'container.dumper.inline_class_loader' : null,

along with container.dumper.inline_class_loader set to true.

Thanks Nicolas for the fix. So the idea is that the AppKernel.php codes enables it, and behind the scene it's disabled for PHP < 7. Guess that's alright for such a "not-well-know" optimization parameter, but still I find it a bit strange. We do have PHP 7 conditions in app.php/app_dev.php so it's explicit. Why not in AppKernel too ?

Thx for the fix @nicolas-grekas. We just crashed the all Meetic website due to this error. :-) I guess we will wait for the 4.4.2 to be release before upgrading to sf 3.4.

@seblegall You mean 3.4.2, right? 😄

Yeah, of course. ;-)

Right after installing a SF 3.4.1 it worked fine, but only after a server restart everything crashed getting the same error!

You can upgrade to symfony3.4.2 and resolve the problem

I just removed the files in var/cache/prod and var/cache/dev inside the project folder. It worked for me