In some distributed environments and with a lot of configured users and connection to Microsoft active directory configured in Check_MK times to distribute the changes from the master to all slave sites grow up strongly. Check_MK distributed monitoring uses Apache web server acting as reverse proxy. Sometimes one of Apache web server’s instances doesn’t return data within a pre-defined time range. In such cases you can get the error like this while trying to replicate your slave with the master site:

HTTP Error - 502: 

502 Proxy Error

Proxy Error
The proxy server received an invalid
response from an upstream server.
The proxy server could not handle the request POST /your_slave_site/check_mk/automation.py.
Reason: Error reading from remote server

In this case the following change helped me. I edited the file /omd/sites/your_slave_site/etc/apache/proxy-port.conf and changed the line that contains the directive ProxyPass from

ProxyPass http://127.0.0.1:5000/your_slave_site retry=0 disablereuse=On

to

ProxyPass http://127.0.0.1:5000/your_slave_site retry=0 disablereuse=On acquire=10000 timeout=600 Keepalive=On ttl=120 connectiontimeout=10

After I saved the edited file I had to re-start the main Apache web server by executing the following command:

service httpd restart

If you have more slaves, you have to repeat the procedure for each of them.