The destination configured from the SSB is unreachable, errors in the logs show connections to the destination failing:
syslog-ng: Syslog connection failed; fd='1', server='AF_INET(188.8.131.52:5534)', error='Connection refused (111)', time_reopen='60'
Remove or replace the external destination that is unreachable to ensure no further buffering is occurring. For information on processing orphaned queue files please see the following kb:
Please ensure that the external destination is valid and that it can be reached. Change the destination if required to ensure proper throughput.