Make sure that your login session hasn't expired. Log out and login again.
Does this error only occur with a single process or does it occur with every process?
Turn on your web browser's debugger (by pressing F12 on most browsers) and go to the Network
tab and check whether the REST endpoint to export the process is showing an error. If so, then post it.
Also add the following line to your env.ini file
debug = 1
Does that show you an error message?
I have encountered processes which can't be exported for some reason. I found that pools and lanes can cause problems. I had to keep deleting elements until I figured out what caused the problem. Sometimes the only solution is to recreate the process. If you figure out what element is causing the problem, file a bug report about it.