Did you install the designer using SAG installer or directly download it from here? Do you have any fixes installed on your environment? Either way, I recommend checking the latest fixes available first, and installing(or download the new version) them if there are any.
If it doesn’t help, please provide more information. A screenshot of the faulty flow, related server.log entries will help a lot. Please also provide the fix levels of your designer and your environment. I see webMethods.io tag. Can you edit the service using wm.io UI?
With the information we have right now, my initial guess is your local eclipse installation has a problem and reinstalling/redownloading the designer could/would help if all the latest fixes are already installed. You can also try running eclipse with another JVM and see if it makes any difference, though this shouldn’t be the case since the product is already shipped and tested with Azul JVM, it should work with that.