Hello Andreas Weimann,
Hope you are well. For updating the WPDM plugin, Please simply click the Save Settings again for the license key option from Downloads > Settings > License, and then please update the plugin from the wp-admin > plugins panel.
For saving settings again for the license key option from Downloads -> Settings -> License, that will regenerate the access token. Thus the plugin will be updated, hope that, it will solve your issue.
If still, the problem exists then you can also manually update the plugin
Thank you and regards
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