This also started happening to me after upgrading to 2.18.2.
If you open QGIS in the command line, then try to run a GRASS tool from
Processing, you will probably see a message saying that it couldn't find
Because you're using ubuntugis-unstable, you probably have GRASS 7.2, so
this should be the reason fro the error.
Switching from ubuntugis-unstable to ubuntugis-stable, which has GRASS 7.0,
did the trick for me.
Not sure if there is another way to fix this while using ubuntugis-unstable
/ GRASS 7.2, without compiling QGIS from source.
Any ideas ?
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