After enabling some modules on a Drupal 8 development in progress I got a WSOD. Debugging information threw the following issue:
Fatal error: Interface 'GuzzleHttp\Promise\TaskQueueInterface' not found in /var/www/html/galtane-clean/vendor/guzzlehttp/promises/src/TaskQueue.php on line 13
I tried to isolate the problem causing module by enabling the modules one by one on a backup of my database (pre-problem), but the error reappeared immediately after the first try. Then I un-installed this module and the problem was still present leading me to believe it has nothing to do with the modules in the first place, but rather with a process that accompanying the enabling of modules inside of Drupal.
That's as far as my debugging skills reach, apart from Googling, which gave me very little clues. This is one post I found and I tried updating Composer with no effect. If anyone has ideas about this, or could point me in the right direction, would love to hear about it!
Try to remove all vendors and install them again. Seems that you have corrupted dependency of guzzlehttp/promise.