The multiplexer uses a resource called pipe with a limited size (length).
The length is sufficient to hold Messages for around about 800 data splits by Default.
Use mpx.ini parameter "pipe-size" to increase it further optionally:
pipe-size: Number Of Bytes
Number Of Bytes should be at least 20 times the number of desired data splits, that is (maxmem+maxfile)/split.
Example mpx.ini Settings:
This results in a number of data splits of 1200 = (800+1600)/2 and thus pipe-size should be set to at least