This problem occurs on Windows only. Macs and Linux are unaffected.
Occasionally, the installer fails with a messager that begins with "Problem with the post-install step. Installation may not complete." When this happens, it usually just after just running the Otto Uninstaller as part of the process to upgrade Otto.
The uninstaller tries to remove the Otto service, but for unkown reasons it doesn't happen immediately. Instead the Otto Service marked "pending delete". If you run the installer while the Otto service is still marked "pending delete", it will you will get the message described above.
How to fix
Rebooting the server solves the problem in all cases.