Not able to add a Product using Quote Line Editor on SF CPQ

Hi,

I'm working on 'Set Up Your First Subscription Product' trailhead module and I'm not able to add Products on a Quote Line editor. Getting following error message:

["Invalid operator: null [a1168000003SH0JAAW]","(System Code)"]. 

I was able to add the Products without any issues earlier but for some reason I cannot add them now. It doesn't matter what Quote or Product I'm working with. Does anyone have any idea what must be causing this issue in my CPQ-enabled Dev org? Is there any way I can debug this?

I have already raised two cases (00845910 and 00849495) with SF but haven't got any response. Any help will be appreciated. 

Kind regards, 

Shankar.