I don't think it's the CSS. I've been working on that for weeks. I can almost do it with my eyes closed. The error says something's wrong with the vendor extension.
I just tried updating it on production, which is now a much different version number than previously, and it's showing a new error:
Warning: SessionHandler::read(): open(/tmp/sess_14ee992d5be78cdb4fc1374690a0d57b, O_RDWR) failed: No such file or directory (2) in /home/xxxx/webapps/xxxx/vendor/symfony/http-foundation/Session/Storage/Proxy/SessionHandlerProxy.php on line 69
This time, however, that file does exist, so the error is confusing.