I guess it has to do with this commit:
48864d37ce8aa1526ddf6efa9463c847e42b766c removed q3process throughout the project
It looks for
qucsfilter,..., in the PATH.
It is unnecessary to ask users to append the installation path to the PATH.
I believe it should look into this order:
* install prefix, previously
QucsSettings.BinDir + "qucsfilter"
* File > App. Settings > Locations (TODO should enable user to point to the location of tools)