encrypted document appears empty

I am trying to open an encrypted document in Ubuntu 21.04 with frontends that use poppler (eg Okular, etc) and in all of them

1. the frontend does not ask for a password,
2. the document is empty.

Unfortunately I cannot share the document, but I would like to do my best providing debugging information. In particular, is there anything I can do with poppler in the command line that would reproduce this or provide debugging info?

I would do it for the current version (libpoppler107 (= 21.02.0-1)) and also compile master from the git repo.

(Sorry if this information is in the docs or the wiki, I would not find it).

Some output from qpdf --show-encryption:

R = 4
P = -4
User password = REDACTED
Supplied password is owner password
Supplied password is user password
extract for accessibility: allowed
extract for any purpose: allowed
print low resolution: allowed
print high resolution: allowed
modify document assembly: allowed
modify forms: allowed
modify annotations: allowed
modify other: allowed
modify anything: allowed
stream encryption method: AESv2
string encryption method: AESv2
file encryption method: AESv2

pdfinfo:

pdfinfo /tmp/napilista.pdf 
Syntax Error: Invalid encryption key length
Creator:        gÓ0KÖço5
                        ±IÊ«mrŁ
�˚¥ï€€«˛«ŸÆ0÷ÿ¸
               "tC™BFOÖZW�
Producer:       ò:æé˚”¹2®zA{Ö\1ØÛ¨§[XûÌ[]¸Å⁄b‘ÝíGÉmÙµRc§··ñÔ
CreationDate:   H}lqv.GPT"Q=AK%HUo`
Tagged:         no
UserProperties: no
Suspects:       no
Form:           none
JavaScript:     no
Pages:          1
Encrypted:      no
Page size:      841 x 595 pts (A4)
Page rot:       0
File size:      58174 bytes
Optimized:      no
PDF version:    1.7

Firefox (88.0.1+build1-0ubuntu0.21.04.2) opens the PDF just fine, after asking for a password. `