Although one can see a mild pleocytosis in CNS vasculitis the extensive CNS vascular testing seems to be all negative, pointing away from CNS vasculitis.
CSF pleocytosis has a very large number of causes. It must also be interpreted in ratio with the number of red cells (about 1:800) as a traumatic spinal tap with 30,000 red cells/mm would have about 26 white cells/mm just be there being some normal blood there/ Also the type of white blood cells present can give clues to the diagnosis, this is not specified in your post
Some possibilities might that may have evaded detection include sarcoidosis, an indolent chronic meningitis (such as fungal), reaction to a viral meningitis or encephalitis, Lyme disease, a drug induced reaction (actually quite common including drugs like NSAIDS and antibiotics), local inflammation from ie a growth or vascular structure, among other rarer causes
Your symptoms though are not necessarily tied to the CSF WBC , and as without any other abnormalities, it should be interpreted with caution. Good luck