Thanks for the response. Yes, I think it's a debug compiled dll.
In the project properties, on both the Compile & Debug tabs, Configuration is set to 'Active (Debug)'.
In the Configuration Manager, the project (there's only one) has configuration set to 'Debug'.
The Advanced Compiler Settings (after clicking Advanced Compiler Options button on Compile tab) has no optimizations checked, Define DEBUG constant is checked, Define TRACE constant is checked.
Is there anything else to verify? Sorry if this is too much information, thought it couldn't hurt to include it.
One thing I forgot to mention previously is that it was working (I'm pretty sure) and stopped after AutoCAD crashed due to an unhandled exception.