After CRM 2015 Update 1.2 were installed, we started getting the below error in one of our Opportunity Forms on click of Next Stage in BPF.
We were using Legacy Form Rendering, and we observed this issue was not occurring in case of New Form Rendering.
After trying out several options, what worked was removing one of the sub grids used in the hidden tab.
Hope it helps..
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I’ v had similar error  on 126.96.36.199. See exact issue on stackoverflow and how you may overcome it: https://stackoverflow.com/a/47182966/50173