Invalid certificate or unknown issuer thrown behind firewall

The certificate looks good, as long as iam not behind a firewall, if i am, the browser shows me an error which says either unknown issuer or invalid cert.

I’ve checked my website with - everything seems valid

My domain is:

My web server is (include version): IIS 10

The operating system my web server runs on is (include version): Windows Server 2016 Datacenter

My hosting provider, if applicable, is: Contabo

I can login to a root shell on my machine (yes or no, or I don’t know): yes

I’m using a control panel to manage my site (no, or provide the name and version of the control panel): Plesk Onyx

Hi @x1x11x

there is an older check of your domain. Your certificate is new
expires in 88 days	*, - 2 entries

the main domain and every subdomain is secure. No chain error, no mixed content. So I don't see a problem.


Share a screenshot.

PS: * doesn't work with a sub-subdomain like


I’m not at work currently, but i can show you the message:


Looks like there is a Man in the middle.

A firewall or anti virus software that creates an own certificate to read the encrypted traffic.

That’s a problem of that connection / firewall / anti-virus-software, not one of your website.

I’m glad to hear this.

Thank you very much @JuergenAuer

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