This looks like some sort of issue with mamba, the package manager. Could you try re-running the installer with the --conda option, which will cause it to use conda instead of mamba? e.g.:
python fslinstaller.py --conda
P.S. @Steven I suspect that the other issue is simply due to the installation taking longer than people expect as, despite it being reported numerous times, nobody has actually provided me with any further information (the few people who did provide me with a log file were affected by other issues).