This has the same eager enablement problem that I described before.

1. Make a tag.
2. Select an existing tag.
3. Check the 'Force replace existing tag.' checkbox.
4. Delete the name.
5. Uncheck the 'Force replace existing tag.' checkbox.
6. Check the 'Force replace existing tag.' checkbox.
7. The 'OK' button is now enabled, click it.

org.eclipse.team.core.TeamException: Tag  creation failed
	at org.eclipse.egit.core.op.TagOperation.updateRepo(TagOperation.java:83)
	at org.eclipse.egit.core.op.TagOperation.execute(TagOperation.java:64)
	at org.eclipse.egit.ui.internal.actions.TagAction$1.run(TagAction.java:111)
	at org.eclipse.core.internal.jobs.Worker.run(Worker.java:54)