If you have your own vendor/ folder, you need to load the new autoload.php manually in your plugin bootstrapping code. You can get more help on the forum.

Or ask on Discord. :)

I did

It works in LS3

How could it work in LS3 when there's no config for it in composer.json? o0

On discord «It worked for LS5 prior to v5.6.x, too»

5.5.0 is the first version, it doesnt work anymore, 5.4.15 is the last version it does

hm, require_once __DIR__ . '/vendor/autoload.php'; instead of require_once 'vendor/autoload.php'; does the trick
checking again

Calling from where? Maybe it's because we removed third_party/ folder and added vendor/ instead?

we do the call from our plugin-init()-method.
maybe our plugin was loading the global composer.json instead of ours
we do one more check, then I think we can close the issue

@bismark, can we close this ticket?

As per the comments, will be closing the ticket.
Please add any comments in case it should be reopened.
Thanks