We came across this issue.
This is due to RAM management.
When you have a huge application, you try to delete, the app gets loaded to the RAM and then gets deleted. It takes a bit of time. If you refresh in between or move to another app, the QMC engine throws error. (i am not sure, how they handled such exceptions as we are getting the exceptions on the screen directly.)
To work it out, best would be, restart the browser. Give the QMC little time before working.
Ultimately, increase the resources in the server.
Thanks and regards,