@developers

In my error log was another bug in src/Core/Storage/Type/ExternalResource.php.
In line 66 $fetchResult can be empty. That should be caught by another if statement, for example:
if (empty($fetchResult)) { throw new ReferenceStorageException(sprintf('External resource failed to get %s', $reference)); } else { throw new ReferenceStorageException(sprintf('External resource failed to get %s', $reference), $fetchResult->getReturnCode(), new Exception($fetchResult->getBody())); }

This might probably help.