I can understand how difficult it is with no clear message in logging. If you want to can increase the logging level to maximum of debug logging. Maybe that will give you a message in logging.
So you did create second slave site and tried/check if the replication does work from your master? I was not sure from the answer if you did.
Last resort that I can think is, create CheckMK and OMD backup. Create clean new site and restore you backup and check if the problem persist.