Tags can now be supplied when creating a new post, see 29113baf02d11339630698cb726a06bd1f461329
I have tried to reproduce this using the steps, but the error does not occur and the tag is added. Can you try and recreate this again? If it truly is fixed, then this issue can be closed.
Tags seem to be applied and created now, see 29113baf02d11339630698cb726a06bd1f461329
Looks like all the original issues have been fixed. Also, it might be nice if when you clicked "Add" to create a new tag, that tag was automatically applied to the blog.
Bugfix