Metadata Link verification problem

I have a problem with the metadata link verification at my account @mertlresearch@mastodon.social. I have added the back-link with the rel=“me” tag on two sites, that I’m referencing in my profile metadata. For one it is working (http://observe.mertl-research.at), for the other not (http://www.mertl-research.at). Both sites are hosted by the same provider.
I have tried the tip that I found related to this problem (removing the rel-tag, save changes in the profile, add the rel-tag again, save changes in the profile), but the homepage http://www.mertl-research.at hasn’t been verified.

I’m serving the homepage from static html files.

Can you help with a hint where to start the troubleshooting of this problem?

Regards,
Stefan.