I had been trying to evaluate gitui unsuccessfully for a few years until now. Most notably starting gitui in my working repositories showed empty "Status [1]" and "Stashing [4]" tabs. Browsing the history and opening a revision/commit showed an empty Diff for individual files, also. I had been starting gitui with a --logging option and I had been noticing the following entry:

[ERROR] fetch_helper: git error:unsupported mandatory extension: 'link'; class=Index (10)

Searching this project issues and discussions, and more generally online, hasn't revealed a possible cause for me, so I've decided to put some traces here. I've finally identified the cause is my usage of the Git core.splitIndex config option. I have reverted the global setting of this option to its default for some time now and it hasn't been showing in git config --list --show-scope, for example, so it had not been obvious to me that it is still the case my repositories had been still having/using a split-index. In the end I've been able to update my repository indexes like:

git update-index --really-refresh --no-split-index

Now gitui appears fully functional on my repositories. Hope this could be a useful hint for anyone experiencing the same issue.