The error
Bad Request
Your browser sent a request that this server could not understand.
Size of a request header field exceeds server limit.
is a cookie issue in your browser because of PayPal:
www.paypal-community.com/t5/My-Feedback-...en-years/td-p/462983
Clearing the cookies of your browser should definitely fix the problem. If not, please contact PayPal's support for help as we can't do anything for you for that problem.
Regarding the SSL errors that you posted, it comes from your SSL certificate. It's either self signed, not configured properly, or signed by someone else who is not trusted either.
You need to contact your hosting company or SSL certificate provider so that they can help you get a trusted certificate configured on your server.