Recently I have seen several customers have problems after upgrades/splits/maintenance where after they login the homepage is returning not authorized errors or the homepage styling is completely messed up.
I was a little surprised to find that the root of many of these issues was actually based on security. In several cases the upgrades or the HR / CS separation process caused a removal of a permission list from the PSCLASSDEFN table, however, the reference to that permission list on the PSROLECLASS table which ties Roles to Permission lists does not remove the reference. This causes the security to have invalid (null) pointers which causes a serious problems for items that are associated to that security and effects the rendering of the home page among other things.
I found that in my case that after we ran a sysaudit report that the entries that showed up in the Security #31 section were the permission list references that were causing the problem. Once resolved all the renderings starting to perform as expected.