I can't see a "not working". Instead, you have created a lot of certificates. Create one certificate, then install and use it 60 - 85 days instead of wasting resources.
Use one of these, you have all you need.
If the Siteground Tool is buggy or if you use it the wrong way, ask Siteground.
Certificate creation has worked, certificate installation not.
I understand the error message, mentioning too many certificates....
The issue of not having a SSL (Let's Encrypt) has been caused unexpectetly. Let's Encrypt was active for this domain.
So I tried to re-install Let's Encrypt. First I got an error that de domainname could not be determined. All DNS A-records are set properly. (including a wildcard * )
Do you have an advise how this problem can be solved?
I think we believe that this is only under the control of Siteground (which creates and maintains the tools that you're using), which is why you would unfortunately need to check with them. If you can find more technical details "under the hood" and more ability to administer/configure your server in another way (not directly through the Siteground Site Tools), then we might be able to help with that if it's software that we can recognize. But otherwise, we can't really support the Siteground software because we don't know anything about how it's meant to work!