UNO board I2C pullups? Can be enabled/disabled?

I don't think so.
The 20k...50k will be no problem.
The 4k7 is the right value, it should work.

Perhaps you have a few breakout boards with pull-up resistors on every breakout board, the total pull-up could be below 1k. That would be too low.

Perhaps you have long wires, say more than 50cm, and they are close to each other, that could be a problem.

Perhaps the decoupling is not good. The I2C device should have 100nF for the supply voltage.

Perhaps you use a breadboard. They don't make good contact a lot of times.

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