That might be exactly what it is.
-Now, restoring from files is out of question.
- Can I achieve it trying different derivation paths? - restoring the right private key
No, the bug is somehow caused by producing a wallet based from two different wallet files (
with different seeds).
So my suggestion was to get all recoverable wallet files so you can try to restore their seed phrase to the latest version.
Changing the derivation path will only derive a different master key but it's still based from the same seed; not what's described in the issue.
With the current data, I have no other suggestions.