Problem loading X.509 certificate -74

In OpenSUSE Leap 15.2 I do not get the DeckLink Mini Recorder 4K up and running if secure boot is enabled.

During startup the following message appears as soon as the Blackmagic key is enrolled:

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kernel: integrity: Problem loading X.509 certificate -74

Here are some other details:

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> sudo lspci | grep Blackmagic
04:00.0 Multimedia video controller: Blackmagic Design DeckLink Mini Recorder 4K


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> sudo dkms status -m blackmagic -k `uname -r`
blackmagic, 11.6a26: added
blackmagic, 12.0a14, 5.3.18-lp152.63-default, x86_64: installed

> sudo dkms status -m blackmagic-io -k `uname -r`
blackmagic-io, 11.6a26: added
blackmagic-io, 12.0a14, 5.3.18-lp152.63-default, x86_64: installed


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> sudo mokutil --test-key /var/lib/blackmagic/MOK.der
/var/lib/blackmagic/MOK.der is already enrolled


Loading the modules fails:

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>  sudo modprobe blackmagic
modprobe: ERROR: could not insert 'blackmagic': Operation not permitted

> sudo modprobe blackmagic-io
modprobe: ERROR: could not insert 'blackmagic_io': Operation not permitted


The card works fine without secure boot (which is not an option for me).

What could be wrong with the X.509 certificate?