We're close to finding the issue. There's a problem with how the query is being constructed it seams. Beat will know more once he investigates further. At this time no further credentials are needed. It seams to have a lot to do with the timing of the mailer. Specifically "Automatically send message at following date", "Relative to this date of the plans below", and "But do not send message if it's later than" values.
To find the query of the mailer ensure debug mode and maximum error reporting are enabled then click the process URL and remove &format=raw from the URL then press enter in your address bar. At the bottom of the page you should see query debugging. You're looking for queries that look similar to the below.
www.joomlapolis.com/forum/153-professional-member-support/232672-mailers-stopped-working-after-upgrade?start=12#277601
The above query has an obvious issue with the date ranges being impossible to match anyone. If the date ranges look ok in your query then your issue could be elsewhere (typically Access settings of the mailer, conditions, or wrong plan state being checked).