Template Questions after upgrade from 5.1.3 to 5.1.5

[FBlack-HPath]

When we upgraded from 5.1.3x to 5.1.5, suddenly none of our users can log into the site. The login box no longer appears (pops up when they try to click login).

We do have customized templates, but in theory the following are the only templates that had new code merged in with the upgrade:

(Style Manager)
css_b_editor.css
Default Template Updated in vBulletin 5.1.3 Alpha 3 by vBulletin Solutions
Your Custom Template Last Edited in vBulletin 5.1.3 by System
Changes were automatically merged into this template

css_b_tabbed_pane.css
Default Template Updated in vBulletin 5.0.6 Alpha 1 by vBulletin Solutions
Your Custom Template Last Edited in vBulletin 5.1.1 by System
Changes were automatically merged into this template

css_editor.css
Default Template Updated in vBulletin 5.1.5 Alpha 1 by vBulletin Solutions
Your Custom Template Last Edited in vBulletin 5.1.5 by System
Changes were automatically merged into this template

I can't figure out what suddenly 'broke' so I'm thinking we'll have to just rebuild our customized look from scratch, based on the most current basic standard templates, so that we know the code is as close as possible to that is should be.

I guess my first question is, what is the most basic (parent) template we should use to start rebuilding our customization from? There is the Default vB5 Style and there is now Themes / vBulletin Default Theme

[FBlack-HPath]

Something in the upgrade made it so idLoginIframeContainer or .b-menu__dropdown-submenu no longer pop up in the main/top navigation and I just have not been able to figure out the conflict.