Steps before I encountered the issue :

Wrote a document in LibreOffice Writer, exported it directly as PDF.
Opened it in Adobe Acrobat Pro (Windows). Put 2 signature-type text fields for me and my friend to sign (it's a debt acknowledgment paper, needs 2 signatures)

Clicked on "Vérifier et Envoyer" (verify and send button).

Expected Result : send a mail
Observed Result : no mail sent. Error code 401.

I also tried to save as a certified directly from the signature tab but it doesn't work. Doesn't work with 1 signature, doesn't work with 2. The progress bar stops half-way and nothing happens.

My friend also tried on his side and he has exactly the same problem. I'll ask him to upvote this post if he can.