I had the same issue and found a solution:
ssh to your Server as root and do the following:
cd /usr/local/psa/admin/sbin/modules
plesk bin extension -r magicspam
This re-registers the extension and after that it's again available under "My Extensions".
By the way, even before that and even though it's not listed under "My Extensions" it is still running, even after the Plesk update. So it's bit uncomfortable that there is an extension, running in the background where you have no control about anymore after the plesk update.
Cheers
Marcus