I already resolved this, but in case any of you will hit same bug:

I'm running SLES10 SP3 + ZCM 10.3.1
after updating to 10.3.1 I cannot pxe boot clients to maintenance mode.

It seems that /srv/tftp/boot/settings.txt file is missing.
I restored version of settings.txt file from backup and now it working

Haven't checked out system-update.log yet for reason why file is missing.