I had an issue a while back on the PC9 pin going bad on my SKR board so I swapped the pins in the config.h file to PF9. This time around I cannot get the laser to turn on when running a cut with lightburn (the pin was previously working), I have the Inline option selected and have tried different baud rates, I double checked the wiring and tried a different laser board.
I recompiled the config.h again to PC1 and when I start a cut in lightburn the laser will “flicker” but not stay on, power is 100%.
I think I would start looking at the signal levels on PC1, ideally with a scope (the basic ones are so cheap now, under $25, I really can’t see a reason not to buy one), otherwise a meter is a poor second choice. If the signal is there check it at the laser, if it isn’t there check for the enable signal on PB0 and suspect your SKR.
I assume the settings in Lightburn are ok if you had it running before and haven’t changed any settings but I don’t use Lightburn so couldn’t advise you further on that.