background info: we are in the process of upgrading to 6.5 on an AMS hosted environment. They provided a sandbox area for us so we can test our custom code.
problem: After merging our 6.4 and 6.5 code base, I cannot add components anymore.
When clicking on a component, all I get is this:

I also cannot drag-and-drop components anymore.
I'm looking for suggestions on what files/configs to check.
1. I checked the error and project logs but nothing related jumps out.
2. I fixed an NPE error in our custom code (which seems to be unrelated) and reinstalled the new package and it didn't fix it.
Thank you.
edit1: I started encountering this issue in local which could be related to the issue with the AMS sandbox environment.
edit2: Seems my local and AMS sandbox is fixed after reinstalling the package. The weird thing with local is that if I had to restart my computer for whatever, I get the problem but if I reinstall our custom package, it starts working again.