hello mandrix, first please make sure that the date, time & timezone are set correctly on that system. if this doesn't solve the issue (or it is already set properly), a possible solution depends on different factors:

• what is the error code shown under technical details on the error page?

in case the error code equals sec_error_unknown_issuer, please attempt to add an exception on the bottom of the error page in order to inspect the certificate like it is shown on the screenshot attached:

• which issuer information does the certificate contain?

please report back with those two pieces of information. thank you!

The error code is always like this, regardless of site: mail.google.com uses an invalid security certificate. The certificate is not trusted because the issuer certificate is unknown. (Error code: sec_error_unknown_issuer)

In the case of anything google related, there is no choice to continue, just get me outta here!

then the second piece of the information requested (the issuer of the failing certificate) would be paramount... if the procedure doesn't work on google or other domains that use HSTS, please perform the steps on a site which does display the "i understand the risks" section instead.

OK. back in a minute.

Holy smokes it's ESET. I never would have expected that. Not sure how to proceed, I guess I'll need to contact ESET support.

I have several other pc's running the same ESET security suite with no problems, so I'm not sure what's going on.

i'm not using eset myself, so i cannot give you exact guidance, but there seems to be an option within the program to import the necessary certificate into firefox it - otherwise you could also disable ssl scanning in ESET's settings. this is the relevant section from the eset smart security 5 userguide :

For SSL communication to work properly in your browsers/email clients, it is essential that the root certificate for ESET, spol. s r.o. be added to the list of known root certificates (publishers). Therefore, the Add the root certificate to known browsers option should be enabled. Select this option to automatically add the ESET root certificate to the known browsers (e.g. Opera, Firefox). For browsers using the system certification store, the certificate is added automatically (e.g. Internet Explorer). To apply the certificate to unsupported browsers, click View Certificate > Details > Copy to File... and then manually import it into the browser.

(page 64 from http://download.eset.com/manuals/eset_ess_5_userguide_enu.pdf)

Thanks, I'll give it a shot. This is all new to me but I'll give it a shake.

Appreciate your help.

OK I went into ESET and unchecked "Add security certificate to known browsers", exited, re-entered ESET and rechecked the box and viola! It appears to be working again.

It's under Advanced setup/web & email/protocol filtering/certificates for future reference.

Thanks again for your help.