Calibre export to ePub fails: "Failed to initialize plugin: ... DeDRM.zip"

Most of the time, it works to add self-written .odt documents to Calibre and then convert to ePub.

But sometimes it fails:

Failed to initialize plugin: '/home/res/.config/calibre/plugins/DeDRM.zip'
Conversion options changed from defaults:
cover: '/tmp/calibre_4.99.4_tmp__w420e3e/o5xos4_r.jpeg'
verbose: 2
read_metadata_from_opf: '/tmp/calibre_4.99.4_tmp__w420e3e/3szthjo8.opf'
Resolved conversion options
calibre version: 4.99.4

Anybody got any ideas how I can solve this?

Greetings
Res