Adguard causes FireFox (55.0.3 64-bit) error: sec_error_unknown_issuer


New Member
Since I updated my 64bit FireFox to the current version 55.0.3 it is blocking almost any site I try to visit using secure HTTPS. In addition, I'm running AVAST Internet Security. I disabled AVAST' s HTTPS-Scanning (an option of AVAST' s Web-Protection component) but without success. The FoxFire error disappeared not until I shut down the Adguard service and unloaded the Adguard executable completely!
I added some online Adguard filters in the past without having problems at all. Must I check all of them now, or is this a known issue with Adguard's basic implementation when running FF 55.0.3 64bit?
Any hints/help appreciated!

Boo Berry

Moderator + Beta Tester
Close Firefox, open Adguard's settings and go to the Network section and select Reinstall Certificate to reinstall the SSL certificate.


New Member
Some months ago reinstalling the certificate solved my problem (running on one of my elder Windows 7 workstations).
Today I'm facing the same malfunction again on my current Lenovo ThinkPad T350 notebook running a 64bit Win10Pro installation. Again, I have AVAST Premier installed, as usual. All other browsers are running flawlessly(!), but my latest FireFox update does not.

Again, I am receiving this wired error message page telling me that the certificate for the sites I try to visit is not valid. Reinstalling Adguard's SSL certificat does not solve the problem.
What more can I do (at the moment)?

Many Thanx in advance!