Error message when trying to copy content elements

When I try to copy content elements from my "This Month in TYPO3" pages to another page, I get these error message:

1: Attempt to insert a record on page '[root-level]' (0) from table 'tt_content' without permissions. Or non-existing page.

2: SQL error: 'Incorrect integer value: '' for column t3o-prod.tt_content.t3o_feuser at row 1' (tt_content:NEW5efabfacb4463630334722)

Both pages have been created by me and also the content elements have been created by me.

The same error occurs when I try to copy a complete page with all records.

I assume that the TYPO3 log shows a bunch of these errors now.

Use an editor account and try to copy content elements.

id=782 or id=784