sophos utm 9.351-3 (Home use only) is using only 2gb memory after hardware change

until yesterday I used old pc hardware ( intel core2quad, 4gb mem, asus mainboard) for my utm.

sophos was using 3.5 gb memory

because i got some problems: nic defective, unexpected reboots

i bought a Supermicro A1SAM-2550F with 16 gb Memory.

last night i replaced the board. after i configured the nic's everything is workinge fine exept

it uses only 2gb of memory:

I would appreciate any ideas!