It is indeed coming from the Real-TimePlatform.
The memory depends on the number of measured signals, the raster speed and the dureation trigger length in the ControlDesk measurment configuration. On the hardware, a buffer is created to be able to hold one entire trigger shot, which may fail if it does not fit to the RAM.
Often a long trigger duration is used causing this problem. Then, the soultion is to measure continuously, which the user most of the time wants instead.
See http://www.dspace.com/faq?408
15kHz is quite fast for the ds1104, by the way, try with litte # of measurements if it works.
Regards, the dspace support :)
(I answered just because i found this question randomly, please contact support@dspace.de (or your local dSPACE company) if you need us!