OptiPlex 780 USFF - Orange light flashing

Hi!

I have a problem with my OptiPlex 780 USFF:

When i shut it down and unplug AC power cord, and plug the cord in again, it won't start, no matter how much time I wait. After like 10 secs of plugging the cord in, orange light starts flashing on the power button and nothing happens.. So what I have to do is to unplug the cord again, hold power button for like 10 secs to release all the residual power left in the PC, then plug the cord in and yeah, it works without any problem! I can now shut it down and turn it on unlimited number of times, just without unplugging AC cord, so also after every power cut it's a problem and I have to fix the way I said I do now.

Now my question is, could this problem be caused by bad CMOS battery? Why I think of this is because it's probably dead, because my BIOS time and date settings always reset themselves.. So I thought it could be the problem..

I know it's easy to check it, just by replacing the battery but because it's USFF I have to remove the heatsink to get to the battery so it's a little problem for me.. But if there is a possibility that the battery causes this problem, I would try to replace it!

Thanks in advance!