Problem connecting with Desktop App to my nextcloud

I recently installed NC, everything went fine. Connection via browser no problem, connection to OnlyOffice went fine. But i get an error when i try to connect with the most recent Desktop Client on Windows 10 (3.1.2). NC is 20.0.6.
This is the error:

Cannot connect securely to

The issuer certificate could not be found

with Certificate

Organization: Helmholtz Zentrum Muenchen
Unit: MCD
Country: DE

Fingerprint (SHA-256): c4:9a:0d:6e:90:da:8b:d3:fd:7b:58:ea:e3:a6:20:f8:6f:2d:7d:6c:3b:38:ee:02:bc:31:c6:fd:ec:ca:2a:f4
Fingerprint (SHA-512): 5d:ad:6b:56:2a:62:7b:b7:1d:ec:99:cb:9b:3c:48:cf:59:54:18:27:87:48:87:e2:0a:f4:8f:d3:7c:0e:72:92:7e:f8:2a:8d:54:24:33:e4:4e:8a:ef:51:04:97:10:ea:9d:02:62:97:9f:a1:13:74:a5:ed:e9:a5:e8:b0:01:43

Effective Date: Di Jul 21 20:20:11 2020 GMT
Expiration Date: So Okt 23 20:20:11 2022 GMT

Issuer: DFN-Verein Global Issuing CA

Organization: Verein zur Foerderung eines Deutschen Forschungsnetzes e. V.
Country: DE

As said, login via browser without any problem.
SSLchecker (SSL Checker) says everything with the certificate and the certificate chain is fine.

Any idea ? Thanks for any help.

Which browser have you tried? I would assume that the Windows desktop client relies on the Windows certificate cache. If the required intermediate and root certificates are not available there, it could be possible that the certificate chain couldn’t be verified.

Certificate chain:
-> DFN-Verein Global Issuing CA.  
   -> DFN-Verein Certification Authority 2
      -> T-TeleSec GlobalRoot Class 2

BTW, I’ve just tested to access the server over the internet and got an internal server error displayed. There seems to be something else wrong in your server configuration :wink: