[SalesForce] Validation rule is not working on QuoteLineEditor page (CPQ)

Just want to a bit extend the answer of brezeton above.

In your case you need to create another field (kind of duplicate of your original field) and populate it before setting new value in the initial field. To achieve this, use the Price Rule. On your Price Rule use On Calculate or After Calculate event.

I couldn't find in CPQ documentation what rules are run first - Price or Product, but from experience I can be quite confident saying that Price Rules are running before Product Rules.

Then, create a Product rule where you compare two fields - one with old value and second with new value.

Thanks to the Salesforce support I have the answer now.

CPQ only expects either all quote lines to be grouped or none. If the lines are grouped, SBQQ__Quote__c.SBQQ__LineItemsGrouped__c needs to be set to true, otherwise the Quote Lines editor will hang with the error I cited.