Well, hard to tell what to make of it but I thought I put it anyway:
I usually use the sandbox option for on my system this seems to be a more stable solution - memorywise, but I just run into a combination where one plugin, motion2.8(synthedit as far as I remember) that is,
is doing strange things to the undo functionality. For me it looks like as if some of motion’s motions get captured in the undo history, making undo behaving really strange (graying out the undo option until you hover over it, when hitting ctrl-z the status tells you anything from nothing to undo to change value undone, effectivly undoing nothing obvious)
Turning sandbox option off seems to resolve that.
Fwiw, I’m using the latest 2.8 beta, xpsp3.
OK, as a follow up, as stated before I had to turn off the sandbox option in the plugin preferences to be able to use renoise’s undo, now some plugins start crashing on me again (couldn’t load blah, fatal error, don’t ignore bug blah) which in turn would lead to me turning the sandbox option on which in turn would render the undo function useless again. 
And again, jbridge wrapper applied to motion fixed this, no more undo issues in sandbox mode.
Still the following question remains:
Why do I have to use sandbox mode? When I work on a song w/o sandbox option ticked everything runs fine and smoothly until reloading the project. When reloading the project plugins start to fail on me, sandbox ticked->everything loads ok.
Now on rc1, btw.