MC2 suddenly stopped working - An error occurred while adding products

Hello Guys,

We have a site where we are using the MC2 still,
I understand the MC3 is out and working perfectly in most scenarios,
But in this particular site, the MC2 works much better compared to the MC3 in terms of responsiveness, I have reported this before in another post.

and back on the topic,
this site has been using the MC2 since before the 5.5.0 update, and is still on the 5.5 version.
Today the site manager reported that the tablets are all not working,
showing an error as below:

after updating the database, and restarting the server, and network, the issue is still there.
I wonder what might be the cause here?