Tags/list yields 'null' as first tag

Sonatype Nexus Repository
,
1

Hello!
We are running nexus 3.58 OSS and have a hosted docker repository.
While working with skopeo we have a problem with some of our images which we could track down to the following:

If we use the tags/list URL (https://neuxs-server/v2/imagename/tags/list) for the image there is ‘null’ as first entry of the tag list:

{
    "name": "imagename",
    "tags": [
        null,
        "1.0.0-1",
        "1.0.0-10",
        "1.0.0-11",
        "1.0.0-12",
[...]

Looking at the image via “browse” doesn’t show the “null” entry but skopeo inspect docker://nexus-server/imagename:version stumbles with FATA[0000] Error determining repository tags: registry returned invalid tag "": invalid tag format.

Has anybody seen this behavior of nexus before?
Where do those “null” tags come from?
How may I get rid of them?

Thanks for any hint or solution!
ronald