Plugin causes fatal error and breaks WordPress admin area

1. 1
   increasing PHP memory limit

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2. 2
   disabling other plugins one by one to find conflicts

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3. 3
   testing on a staging site first

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4. 4
   Rolling back the plugin to a previous compatible version

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5. 5
   Disabling automatic plugin updates

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6. 6
   Outsourcing the code refactoring or fix to an experienced developer

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7. 7
   Testing fixes and restores using a staging site copy

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8. 8
   Temporarily renaming the plugin folder in the file system to disable it and regain admin access.

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9. 9
   Increasing the PHP memory limit to 512M in the environment settings or configuration files.

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10. 10
    Switching to a newer PHP version within the local server settings.

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11. 11
    Reviewing the server routing and PHP error logs to trace the timeout or fatal error.

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In their words

Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.