There was some talk on this list [1, 2] about altogether removing the dependency on Parenscript. (Indeed, SYMBOL-TO-JS[-STRING] is the only feature that CL-JSON makes recourse to.) I'm in the middle of preparing a set of patches which do that, among other (more substantial) things. But anyway, many thanks for pointing out the change.
From what I gather here this will take you at least a few days.
Can you apply my fix? This problem prevents clbuild users from running Weblocks.
Leslie