Slight investigations with browser dev tools lead to node-red/editor-client/src/js/ui/tab-help.js
where function getNodeLabel(n) method is called with argument n which should have _def property defined but it is undefined. Not sure where from the function call originates.
Probably best to raise an issue when you get an error like that. We can then investigate it.
It does sound familiar, but not sure given you are on 2.0.5.
Please include details about what nodes you have installed, and what you've done prior to this happening. Did you just install something? Upgrade something? Edit something?
(But am on holiday this week so my contributions are limited to answering easy questions from my phone in between naps)
Did the step by step uninstall and after the uibuilder (was 3.something) the error was gone. There was no indications that the node could be reason.
But as my laptop was still unreliable (probably due the experimental drivers I'v installed for testing and the uninstall was kind of dodgy) the issue is now out of question due the fresh windows install and for now everything works as expected.