I've worked with a few project management tools by now, but I can't really say that any of them would fit openclonks needs much better than the others. I just wouldn't recommend trac, but my experiences with that date back quite a while as well. As far as Release Management and Chart Tools go, I've never really made any use of it. Maintaining that stuff usually took more effort than it was worth. A combined timeline from repository, wiki and bugtracker is nice, though.

Mantis seems to support LDAP as some sort of SSO solution. I have not actually used that before, so I am not quite sure whether that works (and MantisBT.org seems to be down for me atm..)

It would be a pitty to lose the bugtracker history :/

PS: (1), (2)

I have recently installed and worked with an instance of Trac, there are multiple plugins for authentication available and it should be possible to integrate them here.

On the one hand I would describe Trac as pretty powerful and extendable, on the other hand I have fought with general performance issues before (like pages requiring upwards of 10 seconds to load). I wouldn't even describe myself as an intermediate Python developer, so I also ran into minor problems when installing plugins/eggs, but things usually worked in the end. It just felt like it required a huge amount of configuring and tweaking.

Trac does have a very powerful bugtracker though and offers tight integration into repositories when referencing commits and several other useful features, like ticket referencing and ticket dependencies (as plugin) and timelines/charts. One should realise that Trac is more of a project management suite as opposed to a real bugtracker, but I guess it could be used as a very decent alternative to Mantis.