Now I've tried to reproduce the issue you mentioned.
But I've not encountered any notifications like that.
The AdGuard version - 3.4 nightly 16 (81-1.5.245cl)
The EDGE version - 45.02.4.4931 (Example)
Did you try to reinstall the security certificate in the Network options?
I have already done this and no changes, in addition, everything worked fine before the last update of Adguard, if I deactivate https filtering and / or ad blocking and / or I stop Adguard, I can Connect to duckduckgo.com.
It appears that AdGuard is blocking access to duckduckgo.com.
After several tests, it seems that it was only duckduckgo.com, if I try to connect directly to another site after having tried with duckduckgo.com, it also blocks but if I reload the url of the other site it passes , so the blocking is limited to duckduckgo.com.
Sorry, it was I who made the mistake in excluding duckduckgo.com, everything went back to normal.