It's hard to make any conclusions about the problem without the specific strength of the solution and your darkroom temperature. The solubility of potassium oxalate is 31.0 g/l at 10C, 36.4 at 20C and 38.9 at 30C. As an example, if your working solution contains more thaqn 38.9 g/l and cools below 20C you will see some crystalization.