Hello, I just upgraded to 1.5.15a and as I always have, after file upload I went to http://mysite/install/ and received this message. Not sure what happened.
If you are already running 1.5.15, all you need to do is upload the files for 1.5.15a and no further action is required. When you see a letter after the version it simply means the download has been patched to resolve a particular issue in the code.