Unfortunately, the problem persists after stable core update 162 : After reboot, the Web proxy service is stopped again…
However, I no longer see this problem in operation since the modification of the cache management parameter (memory cache size: 256 MB to 10 MB - see @flaorus solution) and reactivation of ClamAV.
Problem related to the used / consumed RAM ?
Edit: Since the last update (core update 162), the Web Proxy service stops again while IPFire is running…