Add troubleshooting section for Elastic Agent automatic unenrollment on 401 errors

[Copilot]

Summary

Adds documentation for Elastic Agent automatic unenrollment behavior when receiving 401 errors during check-in. In versions < 8.19.0 and < 9.1.0, agents are automatically unenrolled after 7 consecutive 401 failures. This behavior was removed in 8.19.0/9.1.0.

Changes:

• Added troubleshooting section "{{agent}} is automatically unenrolled after failed check-ins with 401 errors" to troubleshoot/ingest/fleet/common-problems.md
• Updated table of contents with new section reference
• Documented version-specific behavior for versions < 8.19.0 and < 9.1.0
• Provided resolution steps: re-enrollment process and investigating 401 error causes
• Added version-gated admonition explaining behavior change in 8.19.0/9.1.0
• Listed common causes: expired API keys, misconfiguration, authentication issues

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

2. Tool(s) and model(s) used: GitHub Copilot

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Website]: Add troubleshooting section about failed check-ins and 401</issue_title>
<issue_description>### Type of issue

None

What documentation page is affected

https://www.elastic.co/docs/troubleshoot/ingest/fleet/common-problems

What happened?

We should cover the situation that if an Elastic Agent in versions < 9.1.0 or 8.19.0 gets a 401 while checkin in for more than 7 times, it gets auto-unenrolled.

This behavior has been changed on 9.1.0 and 8.19.0, removing the auto-unenrollment.

Additional info

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Website]: Add troubleshooting section about failed check-ins and 401 #3708

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[github-actions]

Vale Linting Results

Summary: 1 suggestion found

💡 Suggestions (1)

File	Line	Rule	Message
troubleshoot/ingest/fleet/common-problems.md	535	Elastic.Capitalization	'is automatically unenrolled after failed check-ins with 401 errors' should use sentence-style capitalization.

[github-actions]

🔍 Preview links for changed docs

• troubleshoot/ingest/fleet/common-problems.md

[vishaangelova]

@lucabelluccini, could you please take a look if this is what you had in mind?

[alexandra5000]

Does the severity warrant 'admonition'? I'm not sure myself, just wondering 🙂

[vishaangelova]

@alexandra5000 I used the ‘admonition’ type of admonition to be able to use a title for the block (“Agents are no longer automatically unenrolled”). I wanted to keep this information under the same heading as the rest of the text, but highlight the difference in functionality in 8.19 and 9.1, and make use of the applies_to in the admonition. So currently it looks like this:

Do you think it should be a different kind of admonition, or not an admonition at all, just part of the text?

[vishaangelova]

Hi, @blakerouse! Would you mind having a look at this small PR to verify its truthfulness? Thank you!