Hello @GrafikPLUS.at,
We understand that encountering technical issues can be frustrating. Would you mind trying to run Illustrator under Safe Mode (Windows / macOS) and checking if it helps? Also, try running Illustrator under a different administrator account (Windows / macOS) and sharing your observations. Additionally, try performing a clean reinstallation of the GPU drivers (Intel / NVIDIA / AMD) and setting the Graphics Preference to High-Performance for Illustrator (https://adobe.ly/3Tm1oz1), and checking for any pending OS updates (https://adobe.ly/3TnKAYt).
If the problem persists, kindly share more details, like the exact version of the OS/Illustrator, system config (CPU/GPU/Memory), details about your workflow, whether Illustrator behaves this way with all files, and a screen recording of the problem (https://adobe.ly/3uU0Wyy), so we can investigate this further.
Looking forward to hearing from you.
Thanks,
Anubhav