Help understanding what this error message means - 'The Sales Quotation object has been modified by another user. Please refresh the object and reenter your changes'.

We have custom events/API calls running in the background I think that's causing this but struggling to find the cause.

I am going from the business opportunity, creating a quote but delivery is greyed out. I then try to modify and see the following error:

Any ideas how I can trace back the problem?