Hi Sukrity. In the end we did not really get to the root cause of it. In itself the file size was not an issue as both a colleague and myself had the issue in the end with the same sort of number as text (with leading zeroes needed). With that field altered the files loaded. with very small files it loaded but big files with that field failed to load. something else was happening.
However we do not no what Adobe was doing differently to that field. Our support could not reproduce it but after he had successfully loaded a test file that failed for more I could then after load it successfully too - as if being in cache in our campaign instance skipped the error.
Not really workable though as a solution. My colleague tried using a work around route outside of our company's Citrix network. This is not ideal - but when we did this, i.e. took out of the equation the slower network and Citrix - all the files loaded. Including those that failed before.
Because we think this is fairly rare we are happy with this work around. Hopefully sharing might save some time for others.
(I still speculate that Adobe campaign is trying to convert that field to another data type and running out of time / memory over the network connection. Given the number of test files and scenarios we went through there must be something there with that field. So if you know anything more find anything please let us know.)