First check if the problem is indeed expired or missing cert:
Open Windows Cert Manager:
Win+R ->certmgr.msc -> enter
Check under "Trusted Root Certificate Authorities" if you can find valid cert for "DST Root CA X3".
If it's missing or expired, get the certificate from nmkd's website.
You can use Firefox to export the needed Certificates.
Just go to https://nmkd-hz.de (https is important), then open site info with:
CRTL+i
Then click "Security" -> "show Certificate".
There you'll see 3 Tabs:
nmkd-hz.de <--- R3 <--- DST Root CA X3
Click on "DST Root CA X3" tab. There you'll have the two options to export/save the certificate as a PEM file. You need the one without the chain. Save it to "whatever.CRT" on desktop, then doubleclick on it and choose "install certificate" it to import it into your windows cert-store as trusted CA.
When you run Certificate manager next time:
Win+R ->certmgr.msc -> enter
The cert "DST Root CA X3" should be now under Trusted CAs
Save the "R3" cert as .CRT file as well (its the Intermediate CA, LetsEncrypt), but you prolly wont need it.
!!
Be wary,
Don't casually install CA certificates on you system unless you know that those can be trusted.
It's always better to download those from official website of the issuer.
It case of "DST Root CA X3" cert its:
https://www.identrust.com/dst-root-ca-x3
(its the same cert as you have exported from
nmkd's website. Instead of exporting it from the nmkd's website you can also copy the text from the webpage above into a .crt file and install it that way. I just wanted to show you the way how to do it from the browser, since we had to check what certs nmkd's website is using)
!!
Try the flowframes installer (use the newest one) and see if it still has SSL errors while trying to download files from nmkd's website.
If its still has errors try installing R3 cert as well and try again.
Let me know if it worked :)