Agent failed at Download: SSL connect error

Hello,

my Agent 2.1.1.1 can't download the packages from my server (2.5.0).

The server log is not reporting any errors, but the client shows:

AGENT =>  Communication Server ask for Package Download
    DOWNLOAD => Package history file successfully cleaned for duplicate IDs
    DOWNLOAD => Metadata file <info> for package <1408603411> is located at <https://brain.xxx .de/download/1408603411/info>
    COM SERVER => Initializing cURL library for getFile
    COM SERVER => Using cURL without server authentication
    COM SERVER => Disabling cURL proxy support
    COM SERVER => Enabling cURL SSL server validation support using CA Bundle <cacert.pem>
    COM SERVER => Sending fileGet request to URL <https://brain.xxx.de/download/1408603411/info>
    WARNING *** COM SERVER => Failed to send HTTP Get request <SSL connect error>
    COM SERVER => Cleaning cURL library
ERROR *** DOWNLOAD => Failed to download Metadata file <https://brain.xxx.de/download/1408603411/info> to <C:\ProgramData\OCS Inventory NG\Agent\download\1408603411\info> 

I checked the ssl cert:

brain# openssl x509 -text -noout -in brain.crt
Certificate:
    Data:
        Version: 1 (0x0)
        Serial Number:
            91:d4:a1:a8:17:f3:f1:df
    Signature Algorithm: sha1WithRSAEncryption
        Issuer: C = DE, ST = Bavaria, L = xxx, O = xxx, OU = brain.xxx.de, CN = brain.xxx.de, emailAddress = *Emails are not allowed*
        Validity
            Not Before: Apr 19 12:33:17 2011 GMT
            Not After : Apr 16 12:33:17 2021 GMT
        Subject: C = DE, ST = Bavaria, L = xxx, O = xxx, OU = brain.xxx.de, CN = brain.xxx.de, emailAddress = *Emails are not allowed*

So the CN from the certificate matched with the server string.

Any hints to figurte out the problem?

Regards

Daniel