We've seen this issue a couple of times, yes. And like you pointed
out, there's no one single good way to solve it, as the reasons for
this can be so many. It just means that dfHack was unable to connect
to DF. Beside the tricks you've already tried, you should try running
a different version of DF. Most new versions are compatible, but the
new d17 experimental thing is not.
Hope that helps at least a little :)
- Jonas