Today I have written two tag descriptions. The way I have done this was by clicking the 'help us create it' link in the tag overview.
A few minutes ago I wanted to describe the 'maven' tag, but now the named link references to https://softwareengineering.stackexchange.com/edit-tag-wiki/@tag.Id. As you see there seems to be a problem with the tag.Id parameter. I have tried this with several other tags which doesn't have a tag summary, all of them references to this URL.
I can still create a tag summary by clicking the 'about the ... tag' in the tag overview and then click the 'Propose Tag Wiki'.
Also I can replace the @tag.Id with the name of the tag within this URL. It redirects to an URL where the tag name is replaced with its id.