After the last update this happened:

/usr/lib/apt/methods/http: relocation error: /usr/lib/x86_64-linux-gnu/libgnutls.so.30: symbol _idn2_punycode_decode version IDN2_0.0.0 not defined in file libidn2.so.0 with link time reference    
Reading package lists... Done    
E: Method http has died unexpectedly!
E: Sub-process http returned an error code (127)
E: Method /usr/lib/apt/methods/http did not start correctly

As per Launchpad bug 1843507, comment #4 worked for me, at least temporarily. 

$ wget http://mirrors.kernel.org/ubuntu/pool/main/libi/libidn2/libidn2-0_2.0.4-1.1build2_amd64.deb
$ sudo dpkg -i libidn2-0_2.0.4-1.1build2_amd64.deb
$ sudo apt-mark hold libidn2-0