TYPO3 Core

I can confirm the bug (on 10.4.8)

Seems like this was broken in #88711
https://github.com/TYPO3/TYPO3.CMS/blob/7b919798c8784ba3a3dd8cb82ab2d8a7e777878a/typo3/sysext/backend/Classes/Form/FormDataProvider/TcaGroup.php#L105

The code only works, if one of the multi-clipboards is used, that means: use list module, select a non-default clipboard and move/copy some records to it.
With the default clipboard, the supposed record uid is only the counter, thus the button tries to insert tt_content_1

Copying the code from below seems to help to properly extract the uid from the clipboard array

// Only some tables, filter them:
foreach ($allowed as $tablename) {
    foreach ($clipboard->elFromTable($tablename) as $elementValue => $recordUid) {
        [$elementTable, $elementUid] = explode('|', $elementValue);
        $record = BackendUtility::getRecordWSOL($tablename, $elementUid);
        $sanitizedClipboardElements[] = [
            'title' => BackendUtility::getRecordTitle($tablename, $record),
            'value' => $tablename . '_' . $elementUid,
        ];
    }
}

also present in 11.5.26