I patched Bullet r2250 as suggested in the following post, and it seems to fix the problem:
http://bulletphysics.org/Bullet/phpBB3/viewtopic.php?f=9&t=6107
For those who can’t compile themselves, here is an updated binary:
http://enn0x.p3dp.com/libpandabullet-r10.zip
Besides this I would suggest using either capsules or sphere for character controllers. Boxes tend to get stuck rather easy.