Channel from the ADH to the source application ran out of northbound buffers (channel overflow). Possible the source application stopped reading or the network is having issues. Or for cases of heavy northbound insert/post traffic, possible you may want to increase number of output buffers on the ADH.
${node}*adh*${route}.route*numIpcOutputBuffers : 400
${node}*adh*${route}.route*tcpSendBufSize : 64240
The description of F56: Host Down is:
F56: Host Down Recover Status send down when rejectOpenRequestsOnOverflow is set to True and service has reached over flow condition.
From this information, the request has been rejected due to the service has reached over flow condition. To get more information, you should submit the question directly to Elektron or TREP team via My Account. They can verify the server log in order to identify the problem.