So 5.4.0 or 5.4.1 introduced a new show stopper bug for me. It's really frustrating when a new version breaks something that used to work and more when the fix won't be introduced in the next version. So my option now is to roll back. I can rollback the code but not the database. Users created, deleted and updated content so I can't just make these disappear. I will have to look at the SQL scrips that ran since the last update and determine if i need to change anything manually. I probably need to change the database/DNN version back so that when i do a new upgrade, it starts properly. I might also use a tool (database diff tool)  to script the differences in database schema and data between two DNN versions. I think red-gate.com has one.

Anyone did a code only, no database, rollback and knows of any gotchas?

Lessons learned: Do extensive testing before upgrading in production. While I did some testing, all in the UI, this bug passed by me. Don't upgrade too early when a new version is out and wait and see if any new bugs are too serious for your installation. Even though new bugs are frustrating, what got me pissed is that the next  DNN version won't get the fix and so I have to spend a big amount of time rolling back stuff. I have spent enough time debugging the source code of DNN, I decided it will save me time to just roll back. This bug also seems to be related to being unable to install a module because I get the same "ThrowKeyNotFoundException() at System.Collections.Generic.Dictionary" error. Looks like an issue with the installer library.

While most users are not affected by this bug, it affects me directly.

 I posted that fix and it works. I am glad that was all the work I had to do!