Clicking on the various tabs to view permissions settings and/or masks (Post/Profile/Misc/Content/etc..) does not work.

There is a javascript error triggered when clicking on any of the tabs.

TypeError: 'null' is not an object (evaluating 'old_tab.className') in permissions.js line 244

The problem appears to be with incorrect data being passed to the swap_option javascript function.

In phpBB 3.0.9, we would see a tab with code like:
<a href="#" onclick="swap_options('0', 'f_mask_i', 'f_maskcategory_i', false, true); return false;">

In 3.1 we see a some '0' values being passed instead:
<a href="#" onclick="swap_options('0', '0', '3', false, true); return false;">

In fact, everywhere f_mask_i or f_maskcategory_i appear in a 3.0.9 ACP permissions page, they are replaced with 0 in the same page in 3.1

For example:
In 3.0.9:
<fieldset class="permissions" id="perm0f_mask_i">
<legend id="legend0f_mask_i">

In 3.1:
<fieldset class="permissions" id="perm00">
<legend id="legend00">