Hello ;
Problem looks like blocked host and then gets connection error.
https://dev.mysql.com/doc/refman/5.0/en/blocked-host.html
how many user it has in subscriber table or did you check data of
subcribers. Maybe a wrong character makes connection errors so connection
lost.

Thanks for help. We have over 400 users in our authentication database. The calculate_ha1 parameter is set to 1.

Are you suggesting that an unsupported character in the password column could cause this error?

Is there any documentation on what characters can be used?