I like to have a pager plasmoid in the top right corner of my desktop. Since KDE 4.2 it is always scrabled in some way after loggin in. Using the Oxygen Plasma theme it's downsized into tinyness, using other themes it looks messed up and has to be deleted and added again.
Is there some way I can get it to behave?

This sounds like a bug in the way it handles being in a desktop containment. However, it could be corrupted / damaged SVG caches. Try removing all files beginning with plasma-svgelements in $KDEHOME/cache-${'hostname'}/ then logging out and in again.

Unfortunately that didn't make a difference.

Are you able to reproduce under a completely clean, new user?

Yes, I added a new user for (other) testing purposes and it showed the same behaviour.
Actually it already did this since KDE 4.0, but only when using the Silicon theme. Now it does it with every theme I tried.
I was hoping there is a workaround, like forcing it to reinitialize after login via a dbus command.

I do not believe Plasma currently has a particularly good D-Bus interface at this time. Since this is clearly reproducible, please search for, and if one doesn't exist, file a bug on bugs.kde.org.

Could you please post links back here to bugs you find / file as well please so future searchers can track the issue? Thanks in advance.

ok, will do. Bugtracker's database seems to be down currently... Seems like everyone held back their bugreports for the 4.2 release :-P

Somebody else reported the bug:
https://bugs.kde.org/show_bug.cgi?id=182487