Thanks for the detailed report. Check Settings > Models > API Keys and verify that “Override OpenAI Base URL” is turned off - when enabled, it causes Anthropic API requests to fail with this exact error even with valid keys.
The team is working on a permanent fix - this workaround should let you use your Anthropic key in the meantime.
Similar reports: API Keys for ChatGPT and Anthropic Failing and Anthropic models break when Override OpenAI BaseUrl is set