Cookie length should be less than or equal to 4096

Your Kibana does not listen in https, does it? If not, why do you have protocol: https?
Also, check the issuer. Where did you get that from?

Also please let’s try with the latest ROR build and see if the stacktrace is still there and similar:
https://readonlyrest-data.s3-eu-west-1.amazonaws.com/build/1.18.6-pre2/trial/20190911/enterprise/readonlyrest_kbn_enterprise-1.18.6-pre2-20190911_es6.7.1.zip?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAJEKIPNTOTIVGQ4EQ/20190911/eu-west-1/s3/aws4_request&X-Amz-Date=20190911T162956Z&X-Amz-Expires=604800&X-Amz-SignedHeaders=host&X-Amz-Signature=bf9d770c27159076bd86b7a0028b751e64dc82864a13111cf4b1b4312f819c6d