Hello Serhan Polat,
this sometimes happen when the workflow designer has been open for a long time, for example over night or over the weekend.
Seems to be a bug in the tool, but as a simple restart fixes it, it is probably not a development priority. Also, the new Workflow Designer Version will be a website.
If you make sure to always close the application when you are finished with your work or leave for the day, this issue should not occur.
Greetings from Germany,
Simon H. Hellmann
DocuWare System Consultant