Can a slave i2c IC prevent the master from pulling the the signal low?
No this should not be possible. I2c lines are open drain, so interfaces can only force the signal low. 

I would start by looking for shorts on the slave board between the lines and 3v3. The msp430 will only be able to pull down with a limited strength.

The slave ic could be damaged, or configured incorrectly. Some have bootstraps which mux the pins between i2c and spi (spi can drive the lines high). 

The ic you mention is an isolator, which may exhibit bad behaviour if the secondary voltage isn't correct. Check every pin voltage and look for differences between the failed and working example. 

If none of that works I would guess your ic is damaged. The maximum current for side 1 is 3.5mA. If you have exceeded this, or subjected it to overvoltage then the internal protection diodes may have failed short.