A simple link to an http domain will not cause https issues unless it is serving content (gif, jpg, css, any content...etc) sitting on that http domain, which the copyright notice is not doing.
have a look at content on your https page, being served from an http link. especially have a look at your users avatars, user signatures, and even your custom styles. you may also have ads or third-party APIs attempteming to insert insecure http content on to your https pages.
but a simple off-site http url link will not trigger https warnings.
off topic... ubb.threads 7.6.0 was released Febuary 2017. ubbcentral.com was fully upgraded from http to https sometime mid to late 2017. The current version of ubb.threads is 7.6.1.1, which was released January 2018.
Last edited by isaac; 05/22/2018 6:03 PM. Reason: corrected a keyboard autocorrect typo
