You say "on every reboot the screen blanks after 10 minutes. " I assume this means that if you don't change the setting then it will continue blanking every 10mins for the current session; yes ? And, to be clear, are we talking about 10mins of inactivity, and not "10mins regardless of activity" ?
If you choose&save a screensaver (ie not blank) for say 12 mins, does the screensaver start after 10mins or does the screen blank ?
As a comment, 10mins is a very specific amount of time - the sort of explicit timing which suggests something coded in a config file or even a system BIOS.
There are other reports of settings changes not "sticking", not solely in wattOS but I think I've seen other reports in these forums. As I recall (ie no direct personal experience), generally the solution involves modifying a config file. There are people in these forums who can help with that when the source of the problem is determined.