security - Thunderbird Certificate Alert keep popping up

I'm having the issue in the title since months now and is driving me crazy. I keep getting this warning:

which translates to

Add Security Exception

You are going to modify the way Thunderbird identifies this site.

Banks, shops and other trusted public sites won't ask to do this operation.

Server:

Addres: ...... [Acquire Certificate]

State of Certificate:

The site has tried to identify itself using non-valid information.

Uknown Identity

This certificate is not trusted since it's not been verified by a known authority by using a secure signature.

Save exception permanently

[Confirm Security Exception] [Cancel]

I always check the box and hit Confirm but the message keeps popping up. I think this is related to the software Fiddler which uses certificates in order to read HTTPS requests. The problem is that I don't have Fiddler opened when the message pops up.

Here are the details of the certificate.

This is really annoying, can anyone help me please?

Thank you!

Answer

Port 993 is not HTTPS but IMAPS and thus is not related to fiddler. There are several possibilities why the certificate is not acceptable, like wrong time on the system, SSL intercepting firewalls (often in conpanies), malware... If you need more help please add the details of the certificate to your question, especially the trust path.

Edit: since the details of the certificate were provided the cause of the problem is obvious. From the certificate:

Emitter: CN = avast! Mail Scanner Root O = avast! Mail Scanner OU = generated by avast! antivirus for SSL scanning

This means that this is not the original certificate issued by a trusted CA, but a certificate issued by some SSL intercepting software. I assume, that you have Avast! Antivirus installed and this software will intercept SSL connections to scan the traffic for malware. You need to accept this issuer as a trusted CA - please refer to the manual of your virus scanner for more information.

If you don't have the Avast! Antivirus installed than it might be that a malware claiming to be Avast! intercepts the SSL traffic.