Hi Sedat,
I am glad to know 1.8.6 is finally working for you.
That error message denotes a problem with the SSL certificate installed on the
www.skypixels.at website. This does not mean that the certificate is incorrect necessarily, just that there are some discrepancies such as mismatched server names or certificate subjects. That's why one should always use EV (Extended Validation) certificates for websites.
You can disable secure connections for this repository, that is, specify http:// instead of https://, and it should work. Insecure connections are not advisable these days but in this way you'll be able to download updates from this website.