Graphics problem in 24.04 LTS

I installed Ubuntu 24.04 LTS inside a Virtual Box on a new Windows 11 host. Clicking on the File Manager on the Desktop (or on related buttons like trash bin or CD) does not show a normal window but just some colored stripes, sometimes blinking. A screenshot is appended. Other applications (terminal, software center, firefox, ...) run without any problem. Can anybody please give me a hint, where this problem may come from?