I ask some users to install Qcad and they did and had the same problem, one them said me the package came from the Arch and none Arch user report any issue about it, so its more likely to be a Manjaro issue
A workaround would be:
QCAD uses the default file dialog offered by the system. If the file dialog is broken (for whatever reasons), you can force the use of the Qt file dialog instead under:
Edit > Application Preferences > File > Load/Save > Use system file dialog (untick)
I installed qcad from pamac-manager, just to try. I noticed that in the first screen (units setup screen), I couldn’t see anything, as if the dialog stuff were transparent. I saved it as it is, and started the application, I didn’t face what you are mentioning. Could it be because you are using light theme on your system? Please note that I use dark theme on my system.
When you try to open something, and you face the empty dialog, try to select all (Ctrl+A, or with the mouse)… Are the icons transparent ?
kdeinit4 does not work on konsole, maybe it changed a little bit
sudo kdeinit4
sudo: kdeinit4: command not found
Recapping
When I try to create a new folder the io slave error jumps, when I try to save a file with a new name it shows a warning saying it cant create it.
No, just inside Qcad file manager. But I starting to believe this is some kind of issue between modern versions of dolphin and qcad. I could probably get this working on an old version of dolphin .
This is how looks like when I tried this workaround
I know this may be off-subject, and not an answer, but did you consider trying librecad? it is very similar to qcad in concept. At least, try and see if you would face the same problem.
Unfortunately, I am not able to reproduce your error as I am using i3wm. I think qcad and your version of Dolphin are not compatible, or probably there is some missing library.
Thanks for your help, I think Qcad dev doesnt support third party packages so I guess Im on my own right now. I will try later their binary version for linux, or maybe I ll just compile from sources, I still think I’ll get the same results. In any case I know its because some issues between Dolphin and Qcad, my final try will be on a virtual machine in debian with gnome, that should work out the box