Yep, this is the Flash Sync Speed. This is the maximum speed at which you can take flash pictures with a given camera. The sync speed is the fastest speed during which the shutter is completely open. At faster speeds, as the top of the frame is being exposed, the bottom of the shutter is already covering the lower part of the sensor (or left-right or however your particular camera's shutter works). Sync speeds range from 1/125 - 500th; not sure why you'd be seeing exposure issues at such low speeds. Could you have a particular mode (Slow sync?) selected accidentally?
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