There is a bug in bootpeg1.img in MorphOS 3.12, and that makes booting on Peg1 with certain firmware setups to fail. If you're affected by this bug, you'll have to wait for 3.13 or use 3.11.
Thank you I didn't know it. I had plugged my previous monitor and it didn't work indeed.
Then I tried a new Ati Radeon 9250 PCI and it didn't work at all: black screen and no signal, while it can work on a PC Windows. I didn't expect it!