A sudden change in the Filter section can be caused by an accumulation of cache glitches within the system, Daria.
To verify, I suggest clearing your browser's cache so the system can start fresh. Also, ensure that you're using a supported browser and are up to date with the latest version for better performance.
After that, run the reports again to verify if you can select a customer in the Filter section. If the issue persists, I recommend contacting our phone support. They have the necessary tools and expertise to investigate the cause of this unusual behaviour further:
- Go to Help and choose the Search tab.
- In the search field, type in contact support.
- Click the Contact Us button.
- Enter your concern in the box and press the Continue button.
- Choose the Chat with us or Get a Callback.
To learn how and when to contact them, please refer to this article: Get help with QuickBooks products and services.
In the meantime, you might want to utilize the Modern View report and customise it by Customer:
- Run any Report.
- Click Filter.
- In the first drop-down, select Customer.
- Choose Equals in the second field.
- In the last field, select a specific Customer.

Once the issue is fixed, you might want to export these reports for more customisation options.
Feel free to add your comment below if you need more guidance about report views or customer report concerns in QBO.