Macintosh IIfx system 7.5 does not boot, only 6.0.8

Acquired Macintosh IIfx, 20MB memory, 1GB hard drive, ROM SIMM included part number 341S0784.

The problem is that after installing the operating system, starting from version 7 and ending with 7.6.1, the computer freezes at startup on the welcome screen.

The strange thing is that there is no such problem with version 6.0.8, it boots and works.

Tell me, please, what can it be?