Same here, it has stopped working, probably the error is related to git instead of gitk, I can see the following in the package management history:

Upgrade: gitk:amd64 (1:2.34.1-1ubuntu1.12, 1:2.34.1-1ubuntu1.13)

Upgrade: git:amd64 (1:2.34.1-1ubuntu1.12, 1:2.34.1-1ubuntu1.13)

Upgrade: git-man:amd64 (1:2.34.1-1ubuntu1.12, 1:2.34.1-1ubuntu1.13)