At first, I thought that this issue was not caused by us: I installed Lockpoint on a virgin 4.3-beta-1 install, and while the above error message did appear in the logs upon our load (which is something that we'd investigate regardless), the macro browser was still working fine.
However, after a Confluence restart, I was able to see the behavior described here. Lockpoint adds some methods to Xwork, and it looks like the Confluence Xwork handler gets confused if a runtime exception gets thrown in the middle of adding a new Xwork action. The order of execution is apparently significant, since this was an issue for me when Lockpoint was installed on startup but not if it was loaded at runtime.
As for the root cause, this seems to be related to a Confluence API change (of sorts). The error message above is rather unhelpful, but after taking some educated guesses, I think the problem is that the "final" keyword crept into Confluence 4.3's definition of RemoveAttachedFileAction:
public final class RemoveAttachedFileAction extends AbstractRemoveAttachmentAction
Lockpoint adds an Xwork action (/lockpoint/removeattachment.action) using a handler that extends the above class, so the classloader's unhappiness while manipulating the action table seemingly blows other parts of Confluence out of the water. Note that Lockpoint isn't touching anything that is even remotely near the macro browser--it looks like our failed attempt to add an unrelated Xwork action is causing all of the problems.
To me, this suggests two things:
1- Is the final keyword there for a specific reason? If not, doing "final" would save us a bunch of work. 
2- It seems like Confluence might do a better job of catching this exception in the first place (in XWorkModuleDescriptor.addAction?), such that even if a plugin explodes due to a runtime issue, it should still leave the Xwork action table in a consistent state.
I had a chance to check this against 4.3-rc3, and the older version of our plugin no longer causes Confluence to blow up. Thanks, Craig and Brett!