Problem with Roundcube, PGP and Mailvelope

[jahlives]

I use RC 1.6.9 in a docker container. I installed latest mailvelope addon in my browser (Firefox 130.0.1 on Fedora 40) and configured it as described here to use GnuPG. As my encrypted mails seems to be a bit special I use show_pgp_mime-plugin in RC to display the pgp mime part of the message. When I click on a message in RC I get the following error:

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Could not decrypt this message: Error: Unknown ASCII armor type
But when I copy the msg source shown by the plugin

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This is a PGP/MIME encrypted message. You can decrypt the message with addons like Mailvelope to read it in our webmail client.
-----BEGIN PGP MESSAGE-----

hQIMA5uNb4bLt/bIAQ/9H60kZ99jHI5S4oTez2j+XN+7VPSn8ulaVewp6yovtCOm
[...]
-----END PGP MESSAGE-----

and paste it to mailvelope decrypt section the content can be decrypted properly without any issue. So I'm not sure where my problem is: in RC or in mailvelope?

Any idea how to fix that problem?

Cheers

tobi