Paragraphs: referenced entity error on save
User 21 was deleted, but 21 was still stored as the value in the uid column of the paragraphs tables. Changing the uid to that of an existing user (23) solved the problem:

UPDATE paragraphs_item_field_data SET uid = 23 WHERE uid = 21;
UPDATE paragraphs_item_revision SET revision_uid = 23 WHERE revision_uid = 21;
UPDATE paragraphs_item_revision_field_data SET uid = 23 WHERE uid = 21;


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Note: other tables also contain the uid 21, but without a (noticeable) adverse effect.

UPDATE file_managed SET uid = 23 where uid = 21;
UPDATE node_field_data SET uid = 23 where uid = 21;
UPDATE node_field_revision SET uid = 23 where uid = 21;
UPDATE node_revision SET revision_uid = 23 where revision_uid = 21;


And two from contrib:

UPDATE crop_revision SET revision_uid = 23 where revision_uid = 21;
UPDATE redirect SET uid = 23 where uid = 21;