Issue with XSS trusted users when upgrading from 2.5.9 to 2.8.3

Hi,

I am testing upgrading my moodle site from 2.5.9 directly to 2.8.3.
In general, it seems fine.

However, after the upgrade, the "XSS trusted users" in security overview listed thousand some users that have to be trusted (students basically). Although, it was only about 30 users (teachers) in 2.5.9 before the upgrade.

Am I missing something here?

I am running moodle on Ubuntu 14.04.1 LTS with postgres database.
Apart from security settings and disabled badge, we use mostly default settings (including default role permissions).

Best regards,
Voravit T.