Thank you all. Everything seems to be fine.
- @pherjung has Storeman Installer 1.2.2 working nicely on SFOS 4.4.0.58 when installed via the SailfishOS:Chum GUI app, which is manually set to 4.3.0.15. Storeman Installer installs Storeman as it should (thanks to the SailfishOS:Chum testing repo manually set to 4.3.0.15), which in turn is also working fine.
- @ric9k downloaded and installed the Storeman Installer 1.2.2 manually on SFOS 4.3.0.15 and it woks fine. So does Storeman installed by Storeman Installer, too.
If anybody experiences issues, which are not explained here, please speak up, preferably via an issue report at GitHub.