Composer dump-autoload or update results in fatal error on Laravel 5.5

I just found a solution to this problem. Here it is for those who have the same problem.

I have had to delete the directory kylekatarnls located inside my vendor directory then run composer update --prefer-source and after that composer dump-autoload.

Now all is working just fine.

I remove the vendor directory and the composer.lock file. After execute composer install and all works well.

Downgrade Composer to version 1 worked for me.

composer self-update 1.10.22
rm -rf vendor
composer install

It seems like you're using Composer v2. If so, read on...

Composer v2 adds some new functions to their Plugin interface (namely deactivate() and uninstall())

However kylekatarnls/update-helper < v1.2.1 is implementing said interface but does not implement the new functions.

So to fix, you'll need to update kylekatarnls/update-helper to the latest (v1.2.1 at time of writing), which contains a fix (implements the missing methods):

composer update kylekatarnls/update-helper