"Null primary key not allowed."

Currently I am doing some data-cleansing on one of my CB Sites.

However, whenever I delete a data set (i.e. a user), I get a white screen with "Null primary key not allowed."

The data set is deleted, all seems to be working well afterwards, but it's awkward having to go back to ->CB -> Users manually after each delete.

Haven't observed this issue before... Any thoughts will be appreciated.

I'm guessing it's from a plugin acting on user delete event. Do you have any 3rd party User plugins published in Extensions > Plugins?

There's nothing we can suggest with just that error as it provides no hint of where it's coming from. Try enabling debug mode and maximum error reporting in Joomla global configuration then retry to see if any further information is output.