Is it still an issue ? Did you try to open the designer in Incognito mode of the browser ?
Right now every time i get this I have to restart my computed for it to go away.
Unfortunately , my organization has disabled incognito mode.
Sometimes this error is seen if you keep the IICS page idle for long time and the session is expired. If you try to open any asset, you may get that error.
It should be resolved by clearing the cache and re-opening the browser again.
As informed by Prakash, it would have been easier if you had the access to incognito mode. However clearing cache, closing all chrome instances and re-opening it should also resolve the issue.