Then, in the output file, look for where it writes the error message to stderr (file descriptor 2 - the "-w2" option to truss). Look for something like "G R U B 2 i n s t..." - the error message with an extra space inserted between each character.
The failure was likely right before that message was output, and should be in the truss output right above where the error message is.
Ok, the problem was that Solaris was installed using legacy BIOS settings. I booted and installed Solaris from the USB by "boot bios:...".
Some months ago Microsoft sent out an update that set Windows and the whole PC to UEFI. And I could not upgrade my Solaris bios installation from legacy bios to Solaris UEFI installation. I tried lot of stuff. At the end, I reinstalled Solaris 11.3 using "boot UEFI:..." instead. And now it works as all OSes are installed as UEFI. Dont forget to boot the USB stick as "boot UEFI" instead of "boot BIOS".
One caveat though: the common knowledge is to install Windows first, and then Solaris. That did not work, I tried it numerous times. At the end, I first installed Solaris 11.3, and then Windows 10 pro - and everything works fine now.