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

[Burkhard]

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!

 

[Blaz]

Try reinstalling Adguard's SSL certificate.

 

[Boo Berry]

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

 

[Burkhard]

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!