What do different I2C errors mean?

Timeout on I2C Write BUSY

I2C device not responding

Timeout on I2C Write Transmit Mode 2

I'm currently having a hell of a time talking to my I2C digipots - which were working. But the same part that was working is now not working.... I remember having a horrible time making it work before. In both cases, it was like the SPI problem - i was just checking everything over and over and over.

After powercycling the part, it goes to Timeout on I2C Write BUSY. Calling setup makes it go back to timeout on write transmit.

>for (i=0x28;i<120;i++) { try {I2C2.writeTo(i,0);} catch (err) {console.log("not "+i+err.msg);}}
not 40Timeout on I2C Write BUSY
not 41Timeout on I2C Write BUSY
not 42Timeout on I2C Write BUSY
not 43Timeout on I2C Write BUSY
not 44Timeout on I2C Write BUSY
Execution Interrupted.
>I2C2.setup({scl:B10,sda:B11});
=undefined
>for (i=0x28;i<120;i++) { try {I2C2.writeTo(i,0);} catch (err) {console.log("not "+i+err.msg);}}
not 40Timeout on I2C Write Transmit Mode 2
not 41Timeout on I2C Write Transmit Mode 2
not 42Timeout on I2C Write Transmit Mode 2
not 43Timeout on I2C Write Transmit Mode 2
not 44Timeout on I2C Write Transmit Mode 2
not 45Timeout on I2C Write Transmit Mode 2
Execution Interrupted.
> 

Occasionally, I've gotten I2C device not responding. But I have not seen any pattern in when I get that behavior.

10k resistors are connected between +3.3v and scl and between +3.3v and sda, and all 3 address pins, as well as the not-connected pin are grounded.

Any ideas?