Hello Massimo,
I attempted to reproduce this by taking this mavros_msgs/CommandTOL service defintiion. Fortunately it doesn't even need the whole package, since the service stands on its own, so it was a quick generation.
current_dir/
|-mavros_msgs/
|-srv/
|-CommandTOL.srv
And ran this code:
rosshutdown
rosgenmsg(pwd)
addpath('path\to\current_dir\matlab_msg_gen_ros1\win64\install\m')
clear classes
rehash toolboxcache
rosinit
s = rossvcserver('/mavros/cmd/takeoff', 'mavros_msgs/CommandTOL', @(~,~,r)r);
c = rossvcclient('/mavros/cmd/takeoff');
rosservice type /mavros/cmd/takeoff
rosservice info /mavros/cmd/takeoff
call(c)
It didn't have any issues. I also didn't have to edit the path to the struct definition file; "+custommessages" worked fine for me. I suspect that editing that is preventing the call command from working, if it is expecting +custommessages in the package. I'm not sure why your "rosservice" command was failing though. Can you provide the output of "ver" so I can see what update version of R2021a you are working with? There was an issue in the original release of R2020b that could be related, but it was fixed by R2021a.
Also, can you try shutting down (rosshutdown) and restarting (rosinit) ROS on MATLAB, and just communicating between a MATLAB server and client and see if that works, rather than your external server? One possibility is that the service definition may be different on your external system in comparison to MATLAB. It's worth checking that by running this on both MATLAB and your external system:
rosmsg md5 mavros_msgs/CommandTOLRequest
rosmsg md5 mavros_msgs/CommandTOLResponse
They must be the same to communicate.
-Cam
