Steps to reproduce the problem:
1. Access ADempiere 3.8 (latest version) via: http://postgres.adempiere.io/webui/index.zul
2. Login with: Username: SuperUser; Password: System;
3. Use the default value in the next screen and proceed by clicking "Tick" button;
4. Open up windows from the ERP (by selecting from the menu tree from the left or search in the Lookup field)
E.g. Sales Order window, Invoice (Customer) window.
5. Keep switching between windows, the "Blank Window or Toolbar Issue" will appear;
6. If not, refresh the page and switching between the previously opened windows, the "Blank Window or Toolbar Issue" will appear