Tho old certificate error

Every time pamac-manager wants to update software: Nicht akzeptables TLS-Zertifikat
(unaccaptable TLS certificate) Nicht akzeptables TLS-Zertifikat Nicht akzeptables TLS-Zertifikat
Synchronisiere Paketdatenbanken… Nicht akzeptables TLS-Zertifikat
Fehler beim Synchronisieren der AUR-Datenbank

I already did the pamac-update --force as described, and deleted all occurences of packages-meta-ext-v1.json.gz.

Any chance to get rid of the message?

You could update without using AUR: pamac update --no-aur if your pamac/openssl/curl/ca-certificates… installation is too old.

Otherwise, I can’t see any issue with the server’s certificate. It is still valid until August 26th (Qualys test).
I guess your installation hasn’t seen updates in months.

In any case - though available - custom builds from aur is unsupported