I was just going to respond, but when I came here I found you had already answered
It has happened before that a "backup" copy of the hotpot folder has caused this problem.
An approach that works is to zip the hotpot folder before you upgrade it. You also need to dump your Moodle database (e.g. "mysqldump" for a mysql database), because the upgrade modifies the database tables. Then try the upgrade and if you have problems (very unusual!) you can restore the Moodle database and unzip the hotpot folder, so that you're back where you started.
Anyway, I'm glad to hear you upgrade completed second time round. Now you can enjoy all the benefits HotPot v2.1
all the best