Why does my intervalometer fire only once and then display 99:59:59?

Sounds like a broken unit => RMA | Service center | Get a new one

I don't think there's something you can do about it (if you could, you wouldn't be asking here), especially considering it was working before. This is definitely not a standard behaviour.

Try a 3 second 2 minute 2 second interval, and it should work. The interval has to be a bit longer than the shutter time. The shutter time has to occur in that interval. A one second interval will not do a two second shutter.

EDIT: changed interval, longer than the shutter duration. See comments. This manual posted by twalberg says "at least two seconds".

I had the same issue, replaced quite a few intervalometers and no luck, it turned out that it was not an issues with device but time settings, so if your interval time is less than exposure time this issue happens, basically your interval time should be exposure time + interval, i.e, if exposure time is 3 second and you want another sec exposure shot after one sec, interval should be 4 second so that camera has 3 seconds to finish before looking for interval, if you put interval time excluding exposure time, camera goes in loop as you asked to repeat the action before first action actually finished.

hope this helps.