Same problem here on Debian Buster!
The problem is reported here:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=987187
My computer was like that:
DEBIAN10$ apt policy curl:amd64 libcurl4:amd64 libcurl3-gnutls:amd64
curl:
  Instalado: 7.74.0-1.2~bpo10+1
  Candidato: 7.74.0-1.2~bpo10+1
  Tabela de versão:
 *** 7.74.0-1.2~bpo10+1 100
        100 http://deb.debian.org/debian buster-backports/main amd64 Packages
        100 /var/lib/dpkg/status
     7.64.0-4+deb10u2 500
        500 http://deb.debian.org/debian buster/main amd64 Packages
        500 http://deb.debian.org/debian-security buster/updates/main amd64 Packages
libcurl4:
  Instalado: 7.74.0-1.2~bpo10+1
  Candidato: 7.74.0-1.2~bpo10+1
  Tabela de versão:
 *** 7.74.0-1.2~bpo10+1 100
        100 http://deb.debian.org/debian buster-backports/main amd64 Packages
        100 /var/lib/dpkg/status
     7.64.0-4+deb10u2 500
        500 http://deb.debian.org/debian buster/main amd64 Packages
        500 http://deb.debian.org/debian-security buster/updates/main amd64 Packages
libcurl3-gnutls:
  Instalado: 7.64.0-4+deb10u2
  Candidato: 7.64.0-4+deb10u2
  Tabela de versão:
     7.74.0-1.2~bpo10+1 100
        100 http://deb.debian.org/debian buster-backports/main amd64 Packages
 *** 7.64.0-4+deb10u2 500
        500 http://deb.debian.org/debian buster/main amd64 Packages
        500 http://deb.debian.org/debian-security buster/updates/main amd64 Packages
        100 /var/lib/dpkg/status
Then, I did the trick!
DEBIAN10$ apt reinstall libcurl3-gnutls/stable
And it is ok now! (thanks @Cobalt)
My config after the change:
DEBIAN10$ apt policy curl:amd64 libcurl4:amd64 libcurl3-gnutls:amd64
curl:
  Instalado: 7.74.0-1.2~bpo10+1
  Candidato: 7.74.0-1.2~bpo10+1
  Tabela de versão:
 *** 7.74.0-1.2~bpo10+1 100
        100 http://deb.debian.org/debian buster-backports/main amd64 Packages
        100 /var/lib/dpkg/status
     7.64.0-4+deb10u2 500
        500 http://deb.debian.org/debian buster/main amd64 Packages
        500 http://deb.debian.org/debian-security buster/updates/main amd64 Packages
libcurl4:
  Instalado: 7.74.0-1.2~bpo10+1
  Candidato: 7.74.0-1.2~bpo10+1
  Tabela de versão:
 *** 7.74.0-1.2~bpo10+1 100
        100 http://deb.debian.org/debian buster-backports/main amd64 Packages
        100 /var/lib/dpkg/status
     7.64.0-4+deb10u2 500
        500 http://deb.debian.org/debian buster/main amd64 Packages
        500 http://deb.debian.org/debian-security buster/updates/main amd64 Packages
libcurl3-gnutls:
  Instalado: 7.64.0-4+deb10u2
  Candidato: 7.64.0-4+deb10u2
  Tabela de versão:
     7.74.0-1.2~bpo10+1 100
        100 http://deb.debian.org/debian buster-backports/main amd64 Packages
 *** 7.64.0-4+deb10u2 500
        500 http://deb.debian.org/debian buster/main amd64 Packages
        500 http://deb.debian.org/debian-security buster/updates/main amd64 Packages
        100 /var/lib/dpkg/status
Besides, add the content bellow to the /etc/apt/preferences.d/99debian-backports file, in order to avoid returning to the problem in the next upgrade!
/etc/apt/preferences.d/99debian-backports
Package: *
Pin: release a=buster-backports
Pin-Priority: 900
Package: libcurl3-gnutls:amd64
Pin: release a=stable
Pin-Priority: 901