That error basically means a worker crashed while trying to run the parfor loop. You mention that x is large. If you are using the 'local' cluster, then please be aware that x must be copied to each of the worker processes. This can cause a large amount of memory usage, and it's possible (probable?) that this is causing the workers to shut down.
If you're using a recent version of MATLAB, you might be able to use parpool('threads'), which uses multiple computational threads in a single process, and can avoid some memory duplication. (But not all MATLAB functions can operate in this environment).
Otherwise, you are going to be constrained by the memory on your system. Transferring x to the workers from the client incurs additional duplication while the messages are in transit, so if you're only just exceeding the memory, and you can build x directly on the workers by executing a function, then the following pattern might help:
xC = parallel.pool.Constant(@myFunctionThatBuildsX);