I have aliased qcad to run it from terminal. There is a small glitch when opening a file from a different location using relative path names:

$ alias qcad
alias qcad='/opt/qcad-3.6.0-pro-linux-x86_64/qcad'

$ cd /a
$ qcad some.dwg &
$ cd /b
$ qcad other.dwg

Import Error is shown:
> Can't open file
> /a/other.dwg.
> File is empty.

$ qcad /b/other.dwg # absolute path works

$ readlink /proc/pid-of-qcad/cwd
/opt/qcad-3.6.0-pro-linux-x86_64

Qcad somewhere stores the path, from where it is first launched. The next "launch" will not use the next location but refer to the first one when constructing the full path name of the file name in the argument.

Maybe that can be easily improved (the message is wrong too).