Thanks for your excellent support Mark.
It was definitely a host side issue.
I copied herunder some excerpt from my analysis and the communication with support in order to help anyone with the same issue
From my analysisAll files and directories under :
/[mydomain].com/cache/gantry5
have their rights arbitrarily changed from 755 to 600 for directories as well as from 644 to 600 for the files rights
This halts the gantry5 theme installation scripts with a mkdir error
From support :Hello,
After further checks with a level 2 it could indeed be caused by a problem in the CloudLinux Cage on your account.
Your CageFS was reset for the cPanel user XXXXXX and permissions reset.
Let us know if your concern still persists.
Hello,
All right, thank you for your feedback.
Your ZLE-xxx-yyyyy ticket is related to the permission issue. It has been escalated to level 3.
If you are experiencing further difficulties, please feel free to open another technical ticket.
Our team is always happy to assist you.
Thank you for your confidence in XXXXXXXXX.
The actual soving needed a level3 support escalation !
So prepare yourself to be patient, polite and bullet proof in your argumentation and symptoms explanations to the support team.
They initially "jumped" onto the word " cache" to initally point out a webmaster/CMS/Theme maker malfunction.
It took one week to solve this.