"composer update" causes Illegal string offset 'info'

[hermann77]

composer update drupal/core

causes following error:

Loading composer repositories with package information
https://repo.packagist.org could not be fully loaded (Illegal string offset 'info'), package information was loaded from the local cache and may be out of date

                                
  [ErrorException]              
  Illegal string offset 'info'


Exception trace:
 () at phar:///usr/local/bin/composer/src/Composer/Util/HttpDownloader.php:458
 Composer\Util\ErrorHandler::handle() at phar:///usr/local/bin/composer/src/Composer/Util/HttpDownloader.php:458
 Composer\Util\HttpDownloader::outputWarnings() at phar:///usr/local/bin/composer/src/Composer/Util/Http/CurlDownloader.php:402
 Composer\Util\Http\CurlDownloader->tick() at phar:///usr/local/bin/composer/src/Composer/Util/HttpDownloader.php:404
 Composer\Util\HttpDownloader->countActiveJobs() at phar:///usr/local/bin/composer/src/Composer/Util/Loop.php:96
 Composer\Util\Loop->wait() at phar:///usr/local/bin/composer/src/Composer/Repository/ComposerRepository.php:963
 Composer\Repository\ComposerRepository->loadAsyncPackages() at phar:///usr/local/bin/composer/src/Composer/Repository/ComposerRepository.php:527
 Composer\Repository\ComposerRepository->loadPackages() at phar:///usr/local/bin/composer/src/Composer/DependencyResolver/PoolBuilder.php:379
 Composer\DependencyResolver\PoolBuilder->loadPackagesMarkedForLoading() at phar:///usr/local/bin/composer/src/Composer/DependencyResolver/PoolBuilder.php:234
 Composer\DependencyResolver\PoolBuilder->buildPool() at phar:///usr/local/bin/composer/src/Composer/Repository/RepositorySet.php:261
 Composer\Repository\RepositorySet->createPool() at phar:///usr/local/bin/composer/src/Composer/Installer.php:436
 Composer\Installer->doUpdate() at phar:///usr/local/bin/composer/src/Composer/Installer.php:279
 Composer\Installer->run() at phar:///usr/local/bin/composer/src/Composer/Command/UpdateCommand.php:248
 Composer\Command\UpdateCommand->execute() at phar:///usr/local/bin/composer/vendor/symfony/console/Command/Command.php:245
 Symfony\Component\Console\Command\Command->run() at phar:///usr/local/bin/composer/vendor/symfony/console/Application.php:835
 Symfony\Component\Console\Application->doRunCommand() at phar:///usr/local/bin/composer/vendor/symfony/console/Application.php:185
 Symfony\Component\Console\Application->doRun() at phar:///usr/local/bin/composer/src/Composer/Console/Application.php:336
 Composer\Console\Application->doRun() at phar:///usr/local/bin/composer/vendor/symfony/console/Application.php:117
 Symfony\Component\Console\Application->run() at phar:///usr/local/bin/composer/src/Composer/Console/Application.php:131
 Composer\Console\Application->run() at phar:///usr/local/bin/composer/bin/composer:83
 require() at /usr/local/bin/composer:29