JSON size limit?

First, I love this module, specifically Ajaxsubmit. I use bind it to CCK form with custom code. No problems. However, I have one (huge) CCK form with about 100 fields. When I submit it, Firebug shows this error:

ajaxsubmit.js (line 89)
data.message is undefined
$(this.target).html(data['message']);

I've done a debug in Firebug, but it's slightly too advanced for my jquery / javascript skills. However, the issue is new. It started after I migrated to a new server. I suspect a JSON size limit since there are no problems except for this one (huge) CCK form. Any thoughts? php.ini tweaks? (I already high values already for memory_limit (400M, max_input_time (1000), max_execution_time (1000)).