Aborted connection on an Update statement only
I sort of found the solution:
I tried running the same query directly on a mysql client, and the error was more enlightening: "MySQL error 1449: The user specified as a definer does not exist".

Which led me to this post:
https://stackoverflow.com/questions/10169960/mysql-error-1449-the-user-specified-as-a-definer-does-not-exist

I did apply the proposed solution to "Change the definer". However I was never able to make it work, as the client kept trying to reach the initial definer user (which no longer existed). It seems the user credentials were cached somewhere. So my only solution was to recreate the initial definer. Which stopped the error