Unhandled exception: 400: Site connection not initiated (Heartbeat timeout after 2.0 sec)

Hallo,
appliance latest version
cee 1.6.0p7cee

Configuring a ditributed monitoring ends with

test01 - Livestatus error
Unhandled exception: 400: Site connection not initiated (Heartbeat timeout after 2.0 sec).

Both sites are running on the same appliance.
The site test01 is working running some testchecks.
Has anyone hints?

Thanks
Ralf

xxx
a working connection and the one not working
cl_cc_prod cl_prod
Connect via TCP (IPv4)
TCP address to connect to: 192.168.241.90, 6557
Encryption: Plain text (Unencrypted), Use plain text, unencrypted transport
OnlineOnline Not enabled
test01 test01
Connect via TCP (IPv4)
TCP address to connect to: 192.168.241.90, 6598
Encryption: Plain text (Unencrypted), Use plain text, unencrypted transport
Unhandled exception: 400: Site connection not initiated (Heartbeat timeout after 2.0 sec).Unhandled exception: 400: Site connection not initiated (Heartbeat timeout after 2.0 sec). Not enabled