Jira upgrade is causing authentication failure for incoming emails.

[Olivier Haegi]

What we did:
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Our company maintains a Jira instance on our own infrastructure and we updated our JIRA instance from 10.3.5 to 10.3.8 (and then to 10.3.11).
We followed the documentation 'Upgrading Jira (manual)' (https://confluence.atlassian.com/adminjiraserver103/upgrading-jira-manual-1489807121.html)

Our issue:
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Our Jira server can't receive Incoming emails to create new Issues, new Comments, ...

In Administration > System > Incoming Mail, when we edit an existing Mail server and click the button 'Test Connection'. We get this error:

Unfortunately no connection was possible. Review the errors below and rectify:

AuthenticationFailedException: authentication failure

The error is logged in jirasoftware-home/log/atlassian-jira.log :

Unable to connect to the server at imap.XXXX.XX due to the following exception: javax.mail.AuthenticationFailedException: authentication failure

Workaround:
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When editing a Mail server, if we check 'Change Password' and set the exact same password, the 'Test Connection' button get the message:

The connection was successful.

We have a lot of Mail servers and editing all of them manually is tedious work.

As an alternative we can edit the database directly and it seems also to "fix" the issue.
UPDATE mailserver SET mailpassword='<password>' WHERE name = '<mail server name>';

Question:
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Is something new in the manual update that can explain this authentication failure between the version 10.3.5 and 10.3.8 ?

We suspect the passwords are somehow not 'well' encrypted in the database but we are not sure.
We are for now clueless on what we need to change in our Jira configuration to solve this issue properly.