Hi!
Something similar happened to me trying to send sysex over my acces virus c (no usb connection, only midi DIN). I lowered the BPMs of the track’s or project on my sequencer and it get fixed! Maybe the data you try to send is arriving to the synth faster that it can “accept” that.
Some sysex managers (mine is SYSEX Librarian) have tools in their preferences to address this problem, like “speed transmit” conf or “pause between played messages”. So you can match old synths pace, like I did with the C. Hope that helps someone! 
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