LED flicker when dimmer at and near full brightness setting.

A Lutron dimmer has begun to cause LED bulb flickering at and near full brightness setting.

The dimmer switch has a brightness level setting slider at the right side of the on/off paddle switch. The brightness setting is usually set to maximum. Until now, this dimmer has been flawlessly controlling 6 small LED bulbs. Now, the LED bulbs are flickering at and near full brightness. The flickering isn't at a continuous frequency, it is sporadic. If I set the brightness to 3/4 or lower there's no flickering. I have "exercised" the dimmer level slider by repeatedly sliding it up and down between minimum and maximum then leaving it at maximum brightness but that hasn't stopped the LED bulb flickering.

What is the cause of this problem?

Many things could be the cause of this problem. With LED dimmer issues, there's no one given solution to every problem. I think you're describing what's known as "shimmer" where the brightness of the bulb sporadically changes. If you changed the bulbs, the new ones could have different (inferior) control circuitry. It is also possible that the dimmer was not designed with LEDs in mind. To answer your question with any level of accuracy, you'll need to provide the model number. If you remove the wallplate and dimmer from the wall, there will be a sticker on the side with the model number. It would also be helpful to give the manufacturer and model number of the LEDs. Sliding the control up and down is unlikely to do anything.

TL;DR: Can't tell you without model no. of dimmer and bulbs.