Thank you,
the agent output has this new entries:
RLZP-VEEAM001 48:DF:37:58:C2:68 Microsoft Network Adapter Multiplexor Driver #3 iSCSI - VLAN 166 2 10000000000 {C9295FAE-C3FA-43D5-9E5E-0ABDC0AB3934}
<<< DEBUG1>>>
<<< DEBUG2>>>
<<<check_mk:cached(1589353115,3600)>>>
AgentUpdate: last_check 1589353113.49 last_update 1588666314.0 aghash a27cb29008349537 pending_hash None error None
<<<>>>
<<>>
1589354056
Oh my god. I think I found the Error. Maybe.
Could it be that (Agent Output)
WorkingDirectory: C:\Windows\system32
ConfigFile: C:\Program Files (x86)\checkmk\service\check_mk.yml
LocalConfigFile: C:\ProgramData\checkmk\agent\check_mk.user.yml
AgentDirectory: C:\Program Files (x86)\checkmk\service
PluginsDirectory: C:\ProgramData\checkmk\agent\plugins
StateDirectory: C:\ProgramData\checkmk\agent\state
ConfigDirectory: C:\ProgramData\checkmk\agent\config
TempDirectory: C:\ProgramData\checkmk\agent\tmp
LogDirectory: C:\ProgramData\checkmk\agent\log
SpoolDirectory: C:\ProgramData\checkmk\agent\spool
LocalDirectory: C:\ProgramData\checkmk\agent\local
Could it be, that the 32bit Client is installed? The Veeam Plugin says, that the x64 Agent is required.