Google console error: URLs for your site and alternative URL

My site wind-rose.co.za is multilangual and Google Console / Webmaster reports errors on /de and /nl version although searching for a solution, there is nothing that helps me in the right direction to get this error away. Website is running fine without issues so maybe there is something in the template that causes this error.
Google warns: We found the hreflang tags on your site, but a few of them have technical problems. Take a look at our hreflang report to see which of your hreflang tags have issues. Until you fix these tag errors, your website will not benefit from additional language or region targeting.

Error:
URLs for your site and alternative URLs in 'de-DE' that do not have return tags.
URLs for your site and alternative URLs in 'nl-NL' that do not have return tags.

Thanks in advance!
Peter

Hi

Try switching to a default joomla template and check if same errors are showing. Personally I've never see such errors, so I'm not sure where can this be coming from. Is there any explanation to what tags are expected ?