I had the same problem this morning deploying to Acquia PROD. No issues on DEV or STG but after deployment to PROD I began to get 503s and this log: Error: Class "Drupal\bootstrap\Bootstrap" not found in include_once() (line 31 of .../themes/contrib/bootstrap/bootstrap.theme)
@tikaszvince, your workaround fixed the issue in my case as well, thanks.
It seems like a race condition to me. When I was testing, I was visiting /user and /user/register and sometimes they'd work and other times were a 503.
I hit this issue after updating to Slick 8.x-2.7
I can confirm the problem does not exist with 8.x-2.6 and slick.js 1.8.1 but after updating to 8.x-2.7 the problem occurs. I think it's related to the commit referenced in comment #7 (https://git.drupalcode.org/project/slick/-/commit/22fd2e3b8e1203489c023f...).
In particular, the change of:
var _element = '.' + _id + ':not(.' + _mounted + ')';
to:
var _element = '.' + _id;
I applied the patch provided in #7 and the error stopped. I was curious if it was an issue with Slick 1.8.1 so I downgraded the module to 8.x-2-6 but the error still occurred. I think this is an issue with the module code and not the Slick JS library.