[wp-trac] [WordPress Trac] #54230: Block based widgeteditor can't update "Not a valid JSON response"

[WordPress Trac]

#54230: Block based widgeteditor can't update "Not a valid JSON response"
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 Reporter:  davidlarssonback   |       Owner:  (none)
     Type:  defect (bug)       |      Status:  new
 Priority:  normal             |   Milestone:  Awaiting Review
Component:  Widgets            |     Version:  5.8.1
 Severity:  normal             |  Resolution:
 Keywords:  reporter-feedback  |     Focuses:
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Comment (by robaxxx):

 Same error here. Am also using IIS as the other two above, but wasn't
 aware that it might be related to that? The issue began with the new block
 widgets editor in 5.8.0, now on 5.8.2

 The error happens when clicking Update in Appearance > Widgets. It does
 not happen when updating Widgets in Appearance > Customise. It also does
 not happen when using the Classic Widgets plugin to disable the Block
 editor.

 Have confirmed with everything disabled and using the 21 default theme.
 Everything else works well. PHP 7.4.13

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Ticket URL: <https://core.trac.wordpress.org/ticket/54230#comment:6>
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