Unable to call PC editor (unknown paragraph format <&1>)

What causes this issue?

The PC editor could not be called because there were unknown character
or paragraph formats.

System Response

the system automatically calls the line editor.

How to fix this error?

Check which character or paragraph formats in the text are unknown. All
formats used in the style or the form of the text must be defined in
the new PC editor.
To change the formats, you can use the syntax check in the editor
(choose Text -> Check -> Single). In the form maintenance you can check
all window texts at once by choosing Form -> Check -> Texts.

Error message extract from SAP system. Copyright SAP SE.