Machines created through Machine Creation Services (MCS) using the feature Machine Creation Services Storage Optimization (MCS I/O) will freeze during the peak hours. 

• New users cannot access the Hosted Desktop and Application.
• Users who are already logged in, cannot use the applications, get a message "There is not enough free memory to run this program. Exit one or more programs, and then try again".

Upgrade to Citrix Virtual Apps and Desktops 7 1903 or later version which have redesigned MCS I/O feature.
For XenApp 7.15, redeploy the catalog without the MCS I/O feature as a workaround.

Instructions: 

While creating the Catalog Un-check below two Cache options

Machines created through Machine Creation Services (MCS) using the feature Machine Creation Services Storage Optimization (MCS I/O) will freeze during the peak hours.  New users cannot access the Hosted Desktop and Application. Users who are already logged in, cannot use the applications, get a message "There is not enough free memory to run this program. Exit one or more programs, and then try again".