Stephane, I am just another technical director using the switcher------
I have never noticed the center LED blinking, and I will be on a switcher tomorrow and I can verify mine, however in the meantime.. You may want to try tagging all the sources, BKGND, KEY 1,2,3,4 all together then pull the T-bar from the top to bottom a couple of times. After doing so --- Untag all sources and see if it is still blinking, sound to me as if the switcher is thinking that it is MID transition on one of the transitional sources.
Another thought - (again I have not paid attention to it) --- Put MLE1 on PGM and see if the center LED on MLE 1 illuminates. It maybe an indicator to which MLE is on PGM. I re-enter my MLE 1 into MLE 2 Keys and I have everything different colors, based on what is on PGM/PRE and re-entered.
I'll load a default switcher setting tomorrow afternoon (cst) and check more if you do not replay to this post informing of a different status on yours.
#Carbonite