Dhivya's answer works for me. I created a new line with the same details as the old one that was having an issue. The old one was deleted and continued with the reconciliation. And it works!

This should rather be a comment to Dhivya's answer instead of a separate one but I do not have enough karma yet to do it.

HI Bariteau, instead duplicating whole statement and reconciling, you can just unreconcile the wrong reconciliation then delete the line from statement and save the record. Again edit the statement and reenter the line and reconcile with correct entry.

Hope this help you!
Thanks and regards,

Dhivya.N

This answer was posted in my  Original Question: https://www.odoo.com/forum/help-1/question/operation-not-allowed-since-your-statement-line-already-received-a-number-you-cannot-reconcile-it-entirely-with-existing-journal-entries-otherwise-it-would-make-a-gap-in-the-numbering-you-should-book-an-entry-and-make-a-regular-revert-of-it-in-case-you-want-to-cancel-it-after-internal-transfer-125703

In the end, the best solution I could find was:

1. Un-reconcile the statement with the bad entry.

2. Duplicate the statement (Actions/Duplicate)

3. Edit the new statement so it's date and description match the original

4. Delete the original statement

5. Run reconcile again - no error

I did as much research as I could and I THINK that this should leave things in a stable state.