Thanks for your reply.
It was the same computer and the same browser.
But one day I was in my companies network and the other day i worked from home office and connected via VPN. Now I am back in my company’s office and still have only the test file that I stored in my home office session. All other documents unfortunately still are lost.
Is it related to browser cache? Maybe browser cache was deleted – cannot remember.
Anyway: After my work seemed to be lost I comfigured plantuml correctly to run on my local computer. So I try to keep work locally.