Hey @valhala welcome to the Community, so great to have you here! 
I am really sorry to hear you're running into this, especially on an app you've clearly put a lot of work into. To address your questions: there's no way to clear, reset, or remove stuck context for a specific app. However, starting a fresh new thread in Assist is worth trying first, it resets the conversation context and may get you unblocked. If even a new thread fails immediately, then your app has simply grown too large for Assist to handle, and unfortunately continuing to use Assist on this app as-is won't be possible.
If that's the case, the good news is you don't have to start from scratch, there's a built-in migration tool that converts your existing classic app to the new Retool app builder. Open the new app builder, select "Convert classic app" in the chat panel, choose your app, and it handles the conversion automatically.
Once you're in the new builder, you can keep prompting the same way you have been. And for a complex app like yours, it's genuinely a better experience, more expressive UIs powered by React that you can edit via prompting, AI-first workflows that reduce time to production, greater flexibility with the broader dev ecosystem, and enterprise-grade governance and security baked in by default.
Let me know how it goes! And if you'd like some hands-on help, come join our live Office Hours, we can walk you through the conversion process and help with any questions along the way! 