Whisperfish registration is broken in SailfishOS 4.2 and 4.3.

Workaround was found at Whisperfish | OpenRepos.net Enable Developer Tools and Untrusted Sources, set an SSH password
Download Whisperfish beta 6, OpenSSL 1.1.1 + 1.0.2 and openssl-libs 1.1.1 + 1.0.2 to your device

Enable developer mode, use the terminal or ssh to your device.

devel-su
pkcon refresh
curl -LO https://openrepos.net/sites/default/files/packages/17011/openssl-1.1.1kgit1-1.7.4.jolla_.armv7hl.rpm
curl -LO https://openrepos.net/sites/default/files/packages/17011/openssl-libs-1.1.1kgit1-1.7.4.jolla_.armv7hl.rpm
curl -LO https://openrepos.net/sites/default/files/packages/1867/harbour-whisperfish-0.6.0-0.beta_.6.armv7hl.rpm
pkcon install-local openssl-1.1.1kgit1-1.7.4.jolla_.armv7hl.rpm openssl-libs-1.1.1kgit1-1.7.4.jolla_.armv7hl.rpm harbour-whisperfish-0.6.0-0.beta_.6.armv7hl.rpm

Run Whisperfish, go through the setup procedure and registration.
Enable the Whisperfish repo in Storeman and update to beta 7.