Good you found the cause!
I had issues with one of my timers for outdoor lights. My CFL lights would occasionally flicker, when I went to LEDs was a frequent pulse. Once I dug into things I found my old timer was totally not compatible with LEDs and was actually a fire hazard. That led to a lot of reading; garage door wasn't LED compatible, several dimmers were not.
Point to all of that is you might want to check around, I bet that either they used a cheap incompatible dimmer or it's wired wrong. I found the manager/owner at BatteriesPlus really knew their stuff about LED switches. The guys at the big box hardware stores had no clue.